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THE  UNIVERSITY  OF  CHICAGO 
SCIENCE  SERIES 


Editorial  Committee 

ELIAKIM   HASTINGS   MOORE,  Chairman 

JOHN  MERLE  COULTER 

PRESTON  KYES 


THE  UNIVERSITY  OF  CHICAGO 
SCIENCE  SERIES,  established  by  the 
Trustees  of  the  University,  owes  its  origin  to 
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treatises  which  attempt  to  cover  several  or  all 
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only  in  scattered  articles,  if  published  at  all.  On 
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They  will  be  written  not  only  for  the  specialist 
but  for  the  educated  layman, 


THE  PHYSIOLOGY  OF 
TWINNING 


/ 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER  AND  TAYLOR  COMPANY 

NEW  YORK 


THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDON 

THE  MARUZEN-KABUSHIKT-KAISHA 

TOKYO,  OSAKA,    KYOTO,    FUKUOKA,    SENDAI 

THE  MISSION  BOOK  COMPANY 

SHANGHAI 


THE   PHYSIOLOGY  OF 
TWINNING 


By 

Horatio  Hackett  Newman 

Professor  of  Zoology,   University  of  Chicago 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


nOMlTY  LIBRARY 

ti.  C  State  Colitgt 


Copyright  1923  By 
The  University  of  Chicago 


All  Rights  Reserved 


Published  January  1923 


Composed  and  Printed  By 

The  University  of  Chicago  Press 

Chicago,  Illinois,  U.S.A. 


PREFACE 

This  book  deals  primarily  with  the  causes  and  conse- 
quences of  twinning.  In  191 7  was  published  a  volume 
belonging  to  this  series  and  written  by  the  present 
author,  entitled  The  Biology  of  Twins.  That  volume  was 
limited  to  twinning  in  mammals  and  was  largely  mor- 
phological in  character.  The  word  "  twins"  was  viewed 
broadly  so  as  to  include  one-egg  (monozygotic)  as  well  as 
two-egg  (dizygotic)  twins.  We  now  realize  that  true  twin- 
ning is  essentially  a  matter  of  the  division  of  a  single  egg 
at  various  stages  of  development  and  that  dizygotic  twin- 
ning is  only  a  case  of  plural  offspring  born  simultaneously. 
The  present  volume  is  confined  to  one- egg  twinning  and, 
instead  of  being  limited  to  mammals,  includes  all  known 
types  of  complete  or  partial  one-egg  twinning  in  the 
animal  kingdom.  In  The  Biology  of  Twins  very  little 
attention  was  paid  to  theories  as  to  the  causes  of  twins 
or  to  the  interinfluences  of  twins  upon  each  other.  One 
definite  theory,  however,  was  advanced  by  the  writer 
to  account  for  the  curiously  unique  process  of  twin- 
formation  in  the  armadillos.  This  theory  was  the  first 
real  causal  theory  of  twinning  based  on  facts.  Inas- 
much as  this  theory  is  now  supported  by  a  great  body  of 
evidence  from  many  sources,  the  writer,  at  the  risk  of 
appearing  too  solicitous  about  matters  of  priority,  feels 
impelled  to  lay  definite  claim  to  its  authorship.  This 
theory  is  the  center  and  substance  of  the  present  book. 
When  used  as  a  working  hypothesis  it  bids  fair  to  explain 
a  long  array  of  peculiar  twinning  processes  in  diverse 


viii  PREFACE 

groups  of  animals.  A  considerable  number  of  interesting 
situations,  such  as  hemihypertrophy  in  man,  bilaterality 
in  echinoderm  larvae,  symmetry  reversal,  double  limbs 
and  tails,  supernumerary  organs,  and  certain  types  of 
tumors,  turn  out  to  be  phases  of  twinning.  This  all 
seems  to  mean  that  twinning  is  a  much  more  general  phe- 
nomenon than  we  have  previously  supposed.  Whether 
it  occurs  in  worms  or  in  man,  it  expresses  itself  in 
the  same  series  of  types;  and  there  is  a  deep-seated 
coherency  and  consistency  about  its  varied  expressions. 
It  is  because  twinning  is  believed  to  be  a  consequence  of 
one  of  the  most  fundamental  of  biological  processes  that 
the  present  volume  has  been  written,  in  the  hope  that 
some  of  the  riddles  of  life  may  at  least  partially  be 
answered. 

I  wish  to  express  my  thanks  to  my  friends,  Professor 
F.  R.  Lillie  and  Dr.  A.  W.  Bellamy,  for  their  help  in 
reading  the  manuscript  and  for  valuable  suggestions, 
and  to  Mr.  Kenji  Toda,  who  has  so  skilfully  drawn 
or  redrawn  the  illustrations. 

H.  H.  Newman 

April  10,  1922 


CONTENTS 

PAGE 

List  of  Illustrations xi 

CHAPTER 

I.  The  Nature,  Scope,  and  Causes  of  Twinning      .  i 

II.  Experimental  Production  of  Twins  in  Starfishes  ii 

III.  Twinning  in  Earthworms  and  Their  Allies  .     .  28 

IV.  One-Egg  Twins  in  Fishes 38 

V.  Double  Monsters  or  Conjoined  Twins  in  Fishes  51 

VI.  Twinning  in  Birds 73 

VII.  Twinning    in    Amphibia,    Reptiles,    and    Other 

Chordates 91 

VIII.  The  Causes  of  Twinning  in  the  Armadillos  .     .  100 

IX.  The  Modes  and  Causes  of  Human  Twinning  .     .  121 

X.  Developmental  Hazards  of  Human  Twins      .     .  135 

XI.  Hemthypertrophy — A  Type  of  Minimal  Twinning 

in  Man 159 

XII.  Symmetry    Reversal    and     Mirror-imaging    in 

Twins  and  Double  Monsters 164 

XIII.  Double  Tails  in  Vertebrates 190 

XIV.  Twinning  (Duplicity)  in  Limbs 193 

XV.  Twinning  as  a  Mode  of  Reproduction       .     .     .  206 

Bibliography 220 

Index 229 


IX 


LIST  OF  ILLUSTRATIONS 


PAGE 


Figs.  1-3.  Various  Types  of  Twin  Larvae  of  Patiria       .  17 

Figs.  4-6.  Common  Types  of  Symmetrical  Twin  Larvae  of 

Patiria 18 

Figs.  7,  8.  Two  Advanced  Larvae  of  Patiria 19 

Figs.  9-14.  Types  of  Blastulae  and  Gastrulae  of  Patiria  .       20 

Figs.  15-17.  Types  of  Starfish  Anadidymi 24 

Figs.  18-21.  Four  Stages  in  the  Transformation  of  a  Twin 

Patiria  Larva  into  a  Single  Larva    ...       26 

Fig.  22.  Typical     Twin     Earthworm,     Allolobophora    sub- 

rubicunda •    .      .      .       32 

Figs.  23,  24.  Germ-Ring  Stages  of  Trout  Twins      ...       41 

Figs.  25,  26.  Autosite-Parasite    Types    of    Trout    Double 

Monsters 47 

Figs.  27,  28.  Typical  " Separate"  Twins  of  Trout     ...       48 

Figs.  29-31.  Various  Types  of  Trout  Anadidymi       ...       54 

Figs.  32,  ^^.  Diagrams  Showing  Mode  of  Embryo  Forma- 
tion in  Fishes 58 

Figs.  34.  Triplet  Chick  Embryo  (after  Dareste)  ....       77 

Fig.  35.  Early  Stage  of  Twinning  in  Chick  Derived  from 

Fission  of  the  Blastoderm 78 

Fig.  36.  Chick  Twins  Resulting  from  Double  Gastrulation       79 

Fig.  37.  Chick  Double  Monster  Due  to  Head-on  Collision 

and  Fusion 80 

Fig.  38.  Chick  Double  Monster  Due  to  Fusion  of  Two  Head 

Ends 81 

Fig.  39.  Triplet  Chick  Embryo  (after  Dareste)    ....  82 

Fig.  40.  Typical  Chick  Anadidymus 83 

Fig.  41.  Typical  Chick  Katadidymus 84 

Fig.  42.  Rare  Type  of  Chick  Twin  Resulting  from  Nearly 
Complete  Longitudinal  Fission  of  a  Single 
Embryonic  Axis •       86 

xi 


xii  LIST  OF  ILLUSTRATIONS 


PAGE 


Fig,  43.  A  Case  of  Autosite-Parasite  Chick  Twins  in  the 

Making 87 

Fig.  44.  Types  of  Frog  Twins 92 

Fig.  45.  Double  Monster  Turtle  (Anadidymus)   ....  95 

Fig.  46.  Early  Stage  of  Twinning  in  the  Armadillo     .      .  109 

Fig.  47.  A  Little  Later  Stage  of  Twinning  in  the  Armadillo  114 

Fig.  48.  Polar  View  of  Armadillo  Quadruplet  Blastoderms  .  115 

0 

Fig.  49.  Diagram    Showing   Streeter's    Conception   of    the 

Mode  of  Twin  Formation  in  Man      .      .      .      .  128 

Fig.  50.  Common  Placenta  and  Hearts  of  a  Pair  of  One-Egg 

Human  Twins 144 

Fig.  51.  Common  Placenta  of  Another  Pair  of  One-Egg 

Human  Twins 145 

Fig.  52.  Example  of  acardius  acormus 152 

Fig.  53.  Example  of  acardius  acephalus 153 

Fig.  54.  Example  of  acardius  amorphus 154 

Fig.  55.  Human  Anadidymus 166 

Fig.  56.  Viscera  of  Foregoing  Anadidymus  Showing  situs 

inversus  viscerum 167 

Fig.  57.  Horizontal  Section  through  Head  of  Trout  Ana- 
didymus    168 

Fig.  58.  Skull  Structure  of  Trout  Anadidymus  ....  169 

Figs.  59-61.  Various  Degrees  of  Heart  Doubling  in  Trout 

Twins 170 

Fig.  62.  Trout  Anadidymus  Showing  situs  inversus  viscerum  172 

Fig.  63.  Double-Monster  Trout  of  Autosite-Parasite  Type, 

Showing  Partial  situs  inversus  viscerum       .     .  173 

Fig.  64.  Bipennaria  Larva  of  Patiria  with  Bilaterally  Sym- 
metrical Hydrocoele  Derivatives 181 

Fig.  65.  Pluteus  Larva  of  Echinus  miliar  is  Showing  Reversed 

Asymmetry  of  Hydrocoele  Derivatives    .     .     .  184 

Fig.  66.  Example  of  Limb  Duplicity  in  an  Insect     .      .      .  193 

Fig.  67.  Diagram  of  Limb  Reduplication  in  Amblystoma  .  199 

Figs.  68-71.  Various  Degrees  of  Duplicity  of  the  Human  Hand  202 


CHAPTER  I 

THE  NATURE,  SCOPE,  AND  CAUSES  OF  TWINNING 

THE  NATURE   OF   TWINNING 

Strictly  speaking,  twinning  is  twaining  or  two-ing — ■ 
the  division  of  an  individual  or  an  organ  into  two  equiva- 
lent and  more  or  less  completely  separate  individuals 
or  organs.  The  term  dichotomy  is  almost  a  perfect 
synonym  for  twinning,  for  it  means  literally  a  process  of 
division  into  two  parts.  When,  as  in  the  specific  twinning 
of  the  armadillos  and  in  the  sporadic  twinning  of  other 
species,  more  than  two  offspring  are  produced  from  one 
egg,  the  condition  may  be  still  called  twinning  because 
the  process  of  dichotomy  is  simply  repeated  two  or  more 
times.  Thus  we  may  have  single  twinning,  double, 
triple,  or  even  quadruple  twinning.  The  most  advanced 
phase  of  twinning  is  that  seen  in  the  South  American 
armadillo  Dasypus  hybridus,  which  undergoes  always  as 
many  as  three  twinning  processes  and  often  proceeds  well 
into  a  fourth,  so  as  to  produce  up  to  twelve  embryos 
from  a  single  egg.  I  do  not  consider  polyembryony  as 
exhibited  among  the  parasitic  hymenoptera  an  instance 
of  true  twinning:  this,  for  reasons  that  will  be  made 
clear  later.  Furthermore,  since  twinning  is  essentially 
a  division  of  one  into  two,  we  are  not  justified  in  retaining 
within  the  category  of  twins  any  cases  in  which  two 
individuals  have  originated  from  two  germ  cells  originally 
separate.  In  this  narrower  sense,  therefore,  twinning 
proper  is  always  monozygotic  or  one-egg  twinning. 


2  THE  PHYSIOLOGY  OF  TWINNING 

Though  thus  limited,  twinning  is  still  a  multifarious 
process:  there  are  many  kinds  of  true  twinning.  If  the 
blastomeres  of  the  two-cell  stage  of  cleavage,  destined 
to  develop  into  the  right  and  left  primordia  of  a  single 
individual,  become  physically  or  physiologically  isolated 
so  as  to  develop  independently  of  each  other,  two  half- 
sized,  but  otherwise  normal,  individuals  result:  these 
are  twins.  If  a  young  blastoderm  becomes  bilaterally 
separated  into  two  more  or  less  equivalent  blastoderms 
and  two  independent  individuals  result,  these  are  twins. 
If  a  single  blastoderm  loses  its  axiate  organization  so 
that  two  equivalent  secondary  points  of  gastrulation  arise 
instead  of  one,  and  two  embryonic  axes  result,  this  is 
twinning.  If  a  single  embryonic  axis  exhibits  a  tendency 
for  the  two  bilateral  halves  to  grow  independently  and 
a  double-headed  or  double-tailed  individual  results,  this, 
no  matter  how  complete  or  incomplete  the  isolation  of 
the  two  sides,  is  twinning.  If  an  appendage  such  as  a 
hand  or  a  foot  becomes  double  instead  of  single,  this  is 
also  a  phase  of  twinning. 

Twinning  is  a  matter  of  great  defmiteness  and  depends 
on  the  bilateral  organization  of  the  embryo.  Rarely  does 
a  division  into  more  than  two  individuals  occur,  unless 
one  of  the  two  redivides.  Triplets  are  a  hundred  times 
as  rare  as  are  twins.  This  very  fact  emphasizes  the  essen- 
tial feature  of  twinning:  its  two-ness.  Moreover,  not 
merely  two  individuals,  but  two  equivalent  individuals 
are  formed,  which  are  nearly  always  symmetrically  placed 
with  reference  to  each  other.  Neither  one  is  the  original 
individual;  neither  one  is  secondary  or  subordinate  to 
the  other;  but  both  are  equivalent  individuals  regener- 
ated from  a  half-embryo. 


JNATUKJL,    S^UrJl,,   AINU    LAU5M   Ur     1  WllNJNlJNlj         3 

Twinning  is  essentially  a  process  of  regulation  or 
regeneration  of  a  whole  individual  out  of  a  prospective  half 
individual.  All  that  is  necessary  in  order  to  get  two 
individuals  instead  of  one  is  to  accomplish  the  division 
of  an  embryo  into  two  equivalent  regions.  Each  region 
is  totipotent  and  soon  reorganizes  its  own  complete 
axiate  relations  and  becomes  a  whole  individual.  When 
the  separation  of  halves  is  incomplete,  we  get,  in  the  case 
of  whole  organisms,  double  monsters;  and  in  the  case  of 
special  organs,  merely  double  organs. 

THE   DISTRIBUTION   OF   TWINNING   THROUGHOUT   THE 
ANIMAL   KINGDOM  AND   ITS    SIGNIFICANCE 

In  his  book,  Materials  for  the  Study  of  Variation, 
Bateson  surveys  the  incidence  of  double  monstrosity  in 
the  animal  kingdom.  We  find  that  it  is  found  most  com- 
monly among  vertebrates,  not  infrequently  among  anne- 
lids and  arthropods,  occasionally  in  echinoderms  of 
various  classes,  rarely  in  coelenterates,  cestodes,  brachio- 
pods,  protozoans.  I  have  never  seen  a  reference  to  a  case 
of  twins  or  double  monstrosity  in  Mollusca,  Nemertinea, 
Nemathelminthes,  Rotifera,  Ctenophora,  or  Tunicata. 
These  latter  groups  are  characterized  by  determinate 
cleavage  in  its  most  definite  form.  In  such  groups  the 
uncleaved  egg  is  already  highly  organized,  each  part 
of  the  egg  having  a  definite  prospective  value  as  an 
organ-forming  region.  Hence,  if  the  blastomeres  are 
isolated  in  two-,  four-,  or  eight-cell  stages  they  are  not 
able  to  produce  whole  individuals,  but  merely  parts  of 
individuals.  If  divided  in  the  two-cell  stage,  one  cell 
will  develop  into  a  left-half  and  the  other  cell  into  a 
right-half  embryo.     It  is  no  wonder  then  that  in  groups 

IT 


4  THE  PHYSIOLOGY  OF  TWINNING 

with  strictly  determinate  cleavage  we  find  no  examples 
of  twinning,  for  twinning  requires  a  totipotency  of  blas- 
tomeres  or  regions  of  the  blastoderm. 

Equally  significant  is  the  fact  that  twinning  is 
especially  characteristic  of  the  vertebrates,  a  group  in 
which  cleavage  is  most  clearly  indeterminate,  in  the  sense 
that  the  early  cleavage  cells  appear  to  be  totipotent,  i.e., 
each  able,  if  isolated,  to  produce  a  whole  individual. 
In  the  vertebrates,  although  the  axes  of  polarity  and 
symmetry  are  already  laid  down  in  the  undivided  egg 
and  the  axes  of  the  embryo  tend  to  coincide  with  those 
of  the  egg,  it  appears  to  be  relatively  easy  to  break  down 
the  axiate  relations  either  in  the  egg  or  in  various  embry- 
onic stages,  and  then  to  establish  new  axes.  This  all 
means  that,  apart  from  their  axiate  integration,  the 
blastomeres,  until  a  relatively  late  period  of  cleavage, 
are  potentially  equivalent,  no  blastomere  containing  any 
unique  organ-forming  material,  but  each  being  essentially 
germinal  in  character  and  able  under  the  proper  condi- 
tions to  become  a  new  apical  point  and  to  establish  new 
axes  of  polarity  and  symmetry.  This  extreme  versatility 
of  the  elements  of  the  vertebrate  blastoderm  is,  in  my 
opinion,  the  secret  of  its  twinning  capacity.  There  is 
a  good  reason  then  why  the  vertebrates  are  par  excellence 
the  favored  group  for  twins. 

The  other  group  in  which  twinning  occurs  readily 
is  that  of  the  echinoderms,  in  which  cleavage  is  also 
decidedly  indeterminate.  In  the  annelids  and  arthro- 
pods, where  partial  twinning  occurs  only  occasionally  or 
is  confined  to  a  few  types,  we  find  that  cleavage  is  strictly 
intermediate  between  the  extreme  determinate  and  the 
extreme  indeterminate  types.     Thus  again  the  parallel- 


NATURE,  SCOPE,  AND  CAUSES  OF  TWINNING       5 

ism  between  the  mode  of  cleavage  and  the  incidence  of 
twinning  is  confirmed. 

THE    CAUSES    OF   TWINNING 

For  a  long  time  I  have  held  the  view  that  twinning 
is  essentially  a  phenomenon  involving  a  physiological 
isolation  of  equivalent  parts  of  the  blastoderm  and  a 
regulation  of  the  isolated  or  twinned  regions  into  complete 
embryos.  As  to  the  cause  of  physiological  isolation, 
I  have  maintained,  for  the  armadillo  at  least,  that 
the  essential  feature  is  temporary  cessation  or  radical 
retardation  of  development  at  a  critical  period.  It  is 
not  especially  important  for  the  theory  to  determine  the 
exact  cause  or  causes  of  the  lowered  developmental 
rate,  for  we  know  that  a  great  many  agencies  give  the 
same  end-result  so  long  as  they  accomplish  a  retardation. 
What  retardation  evidently  does  to  the  egg  or  embryo 
is  more  or  less  completely  to  disorganize  its  integrational 
relations,  or,  in  a  word,  to  deaxiate  it.  If  through 
retardation  the  polarity  of  the  egg  or  embryo  becomes 
so  weakened  that  there  are  no  relatively  high  or  low 
metabolic  regions,  all  parts  of  the  embryo  are  left  on  a 
parity;  so  that,  when  normal  developmental  conditions 
once  more  appear,  any  part  of  the  embryo  may  become 
the  apical  point  and  a  new  gradient  will  be  established. 
What  usually  happens,  however,  is  that  enough  of  the 
original  axis  of  polarity  persists  to  permit  just  two 
equivalent  points  on  the  axis,  at  equal  distances  from  the 
original  apical  point,  to  remain  in  such  a  condition  that 
they  are  favored  when  developmental  conditions  return, 
and  these  become  the  growing  points  or  apical  ends  of 
twin  embryos.     Very  commonly  the  developmental  re- 


6  THE  PHYSIOLOGY  OF  TWINNING 

tardation  does  not  seriously  affect  the  blastoderm  until 
the  process  of  gastrulation  begins,  a  process  of  extreme 
delicacy,  as  has  often  been  pointed  out.  Once  fairly 
started  the  cleavage  process  seems  to  be  easily  accom- 
plished even  under  great  difficulties.  This  was  clearly 
brought  out  by  the  writer  (Newman,  191 5)  in  his  work 
with  teleost  hybrids. 

Almost  any  cross,  even  those  between  the  most 
distantly  related  species,  will  go  through  cleavage  nor- 
mally or  nearly  so,  but,  because  of  the  relative  slow- 
ness of  the  cleavage  process,  the  important  crisis  of 
gastrulation  is  the  usual  halting-point  for  such  hybrid 
embryos:  their  developmental  momentum  is  insufficient 
for  them  to  accomplish  gastrulation.  Now  gastrulation 
results  in  the  establishment  of  the  embryonic  axes, 
including  the  axis  of  polarity  and  the  axis  of  symmetry. 
The  result  is  that  if  gastrulation  is  only  temporarily 
halted  the  process  of  formation  of  the  axis  of  symmetry, 
or  the  bilateral  axis,  may  be  so  interfered  with  that  the 
primordia  of  the  two  equivalent  halves  of  the  sym- 
metrical embryonic  axis  may  become  more  or  less 
completely  isolated  physiologically,  and  each  half  may 
produce  a  more  or  less  perfect  whole  embryo  in  the  region 
where  the  isolation  has  occurred. 

INTERPRETATION   OF   DOUBLE   MONSTERS    (COSMOBIA) 

My  interpretation,  therefore,  of  double  monsters, 
at  least  of  those  of  a  symmetrical  sort  which  have  certain 
of  their  central  or  axial  structures  united,  is  that  they 
are  products  of  a  partial  twinning  process  involving  a 
separation  of  equivalent  right  and  left  parts  of  an 
originally  single  embryonic  axis.     This  is  opposed  to  the 


NATURE,  SCOPE,  AND  CAUSES  OF  TWINNING       7 

prevailing  theory  that  double  monsters  are  products  of 
the  fusion  of  two  originally  complete  and  separate 
embryos.  This  question  is  quite  crucial  for  our  general 
interpretation  of  the  nature  and  causes  of  twinning. 
It  is  a  time-honored  question  and  has  been  discussed 
pro  and  con  by  leaders  in  the  history  of  zoology  for  nearly 
two  hundred  years.  Important  names  such  as  those 
of  Caspar  Frederick  Wolff,  Meckel,  the  two  Saint- 
Hillaires,  Knoch,  Dareste,  Rauber,  Panum,  and  others 
are  associated  with  the  earlier  phases  of  the  problem. 
Within  recent  times  we  find  such  men  as  Windle,  Gem- 
mill,  Kaestner,  Wilder,  Stockard,  and  others  still  taking 
sides  on  the  vexed  question  of  whether  double  monsters 
are  derived  from  the  division  of  one  or  the  fusion  of  two 
embryonic  axes.  Gemmill  and  Stockard,  among  recent 
writers,  stand  for  the  fusion  of  separate  embryos; 
Wilder  and  myself,  on  the  other  hand,  hold  to  the  view 
that  double  monsters  are  incompletely  divided  single 
embryos.  All  of  these  views  will  be  discussed  in  detail 
because,  to  my  mind,  an  acceptable  theory  of  the 
morphology  and  the  physiology  of  twinning  depends  on 
a  correct  interpretation  of  the  mode  of  origin  of  double 
monsters. 

If  double  monsters  are  merely  the  incompletely 
separated  and  regenerated  bilateral  halves  of  an  originally 
single  embryo,  we  have  a  rational  interpretation  of  situs 
inversus  viscerum  and  mirror-image  symmetry.  From 
this  point  of  view  we  can  proceed  to  an  understanding 
of  various  kinds  of  partial  twinning  and  of  such  allied 
phenomena  as  symmetry  reversal  in  various  asym- 
metrical forms  such  as  echinoderm  larvae.  The  concep- 
tion that  double  monsters   are  products  of   a  partial 


8  THE  PHYSIOLOGY  OF  TWINNING 

fission  is   an   integrating   thread   running   through   the 
whole  fabric  of  our  twinning  theories. 

THE  ORDER  OF  PRESENTATION  OF  DATA 

ON  TWINS 

In  general,  the  order  of  chapters  is  based  on  the 
relative  simplicity  or  the  fundamental  character  of  the 
twinning  processes.  The  simpler  and  more  fully  under- 
stood cases  of  twinning,  such  as  those  in  the  starfishes 
and  earthworms,  come  first  because  it  is  believed  that 
there  the  phenomenon  of  twinning  is  less  obscured  by 
developmental  complexities.  The  most  obscure  and 
least  understood  cases  of  twinning,  as  that  in  man,  are 
reserved  for  later  consideration  in  order  that  we  may 
have  as  much  information  as  possible  about  the  modes, 
causes,  and  consequences  of  twinning  before  tackling 
the  most  difficult  of  our  problems.  In  dealing  with 
twinning  in  the  vertebrate  classes  the  phylogenetic  order 
is  followed  because  this  order  seems  to  lead  from  the 
simpler  to  the  more  complex  phases  of  the  subject. 
The  causes  of  twinning  in  the  armadillos  of  the  genus 
Dasypus  seem  to  me  to  be  more  definitely  known  than 
those  of  any  other  vertebrate,  but  the  restatement  of 
this  situation  is  postponed  until  after  the  consideration 
of  the  twinning  conditions  in  the  birds  and  in  the  reptiles. 

The  study  of  the  literature  on  twins  and  double 
monsters  in  man  brought  to  light  two  rather  unexpected 
and  extraordinary  situations  that  seem  to  me  to  throw 
much  light  on  matters  formerly  quite  obscure  to  biolo- 
gists. The  first  of  these  situations  has  to  do  with  the 
various  developmental  hazards  of  human  twins,  especially 
one-egg  twins,  due  largely  to  interinfluences  of  the  two 


NATURE,  SCOPE,  AND  CAUSES  OF  TWINNIXd       9 

individuals.  The  second  situation,  which  is  technically 
called  hemihypertrophy,  or  a  unilateral  gigantism  of 
one  half  of  the  body,  strongly  suggests  partial  physio- 
logical isolation  of  the  bilateral  primordia,  or  minimal 
twinning.  The  consideration  of  double  monsters  of  all 
classes  has  brought  to  light  the  existence  of  the  phe- 
nomenon known  as  situs  inversus  viscerum  and  other  sorts 
of  mirror-image  symmetry  between  the  two  components. 
This  phenomenon  strikes  deep  at  the  roots  of  the  physi- 
ology of  symmetry  and  asymmetry  in  the  vertebrates; 
hence  a  chapter  is  devoted  to  an  analysis  of  the  observed 
conditions.  After  the  subject  of  twinning  of  whole 
organisms  has  been  concluded  the  discussion  shifts  to 
twinning  in  tails  and  limbs.  Duplicity  in  tails  turns 
out  to  be  merely  a  phase  of  bilateral  twinning  involv- 
ing a  posterior  growing  region,  the  tail-bud.  Twinning 
in  appendages,  however,  seems  to  involve  certain  com- 
plexities that  are  not  present  in  bilateral  twinning.  An 
understanding  of  the  sources  of  complexity,  however, 
seems  to  indicate  that  the  same  fundamental  causes 
and  consequences  of  twinning  hold  good  for  twinning 
or  duplicity  in  limbs  as  for  twinning  of  the  whole  body. 
There  occur,  moreover,  comparable  phenomena  of  sym- 
metry reversal  and  mirror-imaging  that  seem  to  aid  in 
an  analysis  of  the  general  laws  of  symmetry. 

In  a  final  chapter  twinning  as  a  mode  of  reproduction 
is  discussed.  It  is  shown  that  twinning  is  a  form  of 
axiate  reproduction,  that  twinning  is  not  a  reminiscence 
of  a  lost  ancestral  asexual  phase  in  an  ideal  alternation 
of  generations,  and  that  twinning  and  polyembryony 
are  two  entirely  different  phenomena  and  should  never 
be  confused. 


io  THE  PHYSIOLOGY  OF  TWINNING 

Throughout  the  book  an  attempt  is  made  to  put  the 
various  types  of  twinning  in  their  proper  place  in  the 
system  of  biological  phenomena  and  to  show  in  what 
ways  a  knowledge  of  the  causes  and  consequences  of 
twinning  helps  to  advance  our  knowledge  of  several 
obscure  and  difficult  but  none  the  less  fundamental 
aspects  of  animal  biology. 


CHAPTER  II 

EXPERIMENTAL  PRODUCTION  OF  TWINS 
IN  STARFISHES 

INTRODUCTION 

Since  The  Biology  of  Twins  was  written  much  new 
light  has  been  thrown  on  the  nature  and  causes  of 
twinning  and  perhaps  the  clearest  analysis  of  the  funda- 
mentals of  twinning  has  been  obtained  through  the 
experiments  of  the  present  writer  on  the  eggs  and 
embryos  of  the  Pacific  Coast  starfish,  Patiria  miniata. 
It  was  felt  that  an  experimental  analysis  of  twinning 
was  almost  impossible  in  the  case  of  mammals  and  it  was 
therefore  decided  to  make  use  of  some  species  whose 
development  was  well  known  to  be  simple,  readily 
controlled,  and  capable  of  easy  observation.  One 
would  naturally  select  for  such  work  either  the  bony 
fishes  or  the  echinoderms:  two  groups  which  for  a  long 
time  have  been  favorite  materials  for  experimental 
biology.  The  echinoderms  were  chosen  in  preference  to 
the  fishes  partly  because  the  processes  of  cleavage  and 
gastrulation  are  simpler  and  less  open  to  controversy. 
For  a  long  time  I  have  felt  that  twinning  was  intimately 
associated  with  the  process  of  gastrulation  and  on  that 
account  it  seemed  wise  to  study  twinning  in  a  form  in 
which  this  whole  process  is  so  nearly  diagrammatic. 

When  I  went  to  the  Pacific  Grove  laboratory  in  the 
spring  of  1920,  one  of  my  chief  concerns  was  to  study 
experimentally  the  process  of  twinning  in  some  echino- 

11 


12  THE  PHYSIOLOGY  OF  TWINNING 

derm,  but  I  hardly  expected  to  find  so  favorable  a  form 
as  Patiria  y  a  form  that  produces  abundant  twins  almost 
without  any  effort  on  the  part  of  the  investigator. 
Under  ordinary  laboratory  conditions,  and  especially 
under  conditions  of  crowding,  one  gets  a  high  percentage 
of  twins  of  many  sorts.  It  then  becomes  the  task  of  the 
investigator  to  discover  the  factors  responsible  for  this 
high  incidence  of  twins.  Such  an  analysis  has  been  made 
and  is  here  to  be  presented. 

THE   CONDITIONS   UNDER   WHICH   TWINS 
ARE   PRODUCED 

A.  Partheno genetic  twins. — It  was  a  matter  of  some 
surprise  to  note  in  certain  control  cultures  of  Patiria 
eggs,  that  had  never  been  fertilized,  considerable  num- 
bers of  actively  swimming  larvae.  Many  of  these  larvae 
were  of  dwarf  size,  others  were  almost  formless,  wrinkled 
ciliated  masses;  but  there  were  always  some  distinctly 
twinned  larvae,  in  the  sense  that  they  had  two  or  more 
archentera  or  primitive  alimentary  tracts.  Such  twins 
lived  a  long  time  but  were  always  quite  subnormal  with 
respect  to  the  development  of  typical  larval  characters. 
The  percentage  of  parthenogenetically  developing  eggs 
in  Patiria  ranges  from  o  in  some  cultures  to  as  high  as 
75  in  one  culture.  The  usual  percentage  ranges  from 
i  to  10  and  is  more  commonly  about  2  or  3.  Of  the  eggs 
developing  parthenogenetically  as  many  as  a  third,  or 
possibly  a  half  in  some  cases,  show  some  phase  or  other 
of  twinning. 

What  is  there  about  parthenogenesis  that  favors 
twinning?  This  was,  of  course,  the  most  obvious 
question  that  presented  itself,  and  it  was  not  to  remain 


EXPERIMENTAL  PRODUCTION  OF  TWINS         13 

long  unanswered.  One  needed  only  to  watch  carefully 
a  batch  of  unfertilized  eggs  simultaneously  with  another 
batch  of  fertilized  eggs  to  note  certain  obvious  differences: 
(a)  Both  sets  of  eggs  undergo  maturation  at  the  same 
time,  (b)  Numerous  eggs  in  the  fertilized  lot  produce 
well-elevated  fertilization  membranes;  while  in  partheno- 
genetic  cultures  this  happens  in  only  a  few  eggs,  (c)  In 
two  hours  after  insemination  the  fertilized  eggs  begin 
cleavage,  which  goes  forward  at  a  rapid  rate;  while  in 
the  unfertilized  lot  those  eggs  that  had  formed  a  mem- 
brane fail  to  cleave  and  disintegrate  after  a  few  hours. 
(d)  Some  of  the  maturated  eggs  in  the  unfertilized  lot, 
without  having  formed  any  distinctly  lifted-up  fertiliza- 
tion membranes,  begin  cleavage  after  about  six  hours, 
nearly  four  hours  later  than  in  the  fertilized  eggs,  and 
cleavage  goes  on  relatively  much  more  slowly  than  in 
the  latter. 

It  is  obvious  from  these  observations  that  the 
development  of  parthenogenetic  eggs  has  been  greatly 
slowed  down.  The  stimulus  afforded  by  the  entrance 
of  the  spermatozoon  has  been  lacking,  as  is  evidenced 
by  the  absence  of  cortical  changes  and  the  failure  of  a 
fertilization  membrane  to  be  elevated  from  the  vitelline 
membrane.  Moreover,  all  of  the  developmental  changes 
go  on  at  an  abnormally  slow  rate. 

B.  Hybrid  twins. — Twins  of  another  sort,  rather 
better  developed  and  more  viable  than  parthenogenetic 
twins,  were  frequently  found  in  cross-bred  cultures  in 
which  the  eggs  of  Patiria  had  been  fertilized  by  the  sperm 
of  various  other  species  of  starfishes  and  sea  urchins. 
As  compared  with  pure-bred  Patiria  Larvae,  hybrid 
larvae  always  showed  a  very  distinctly  slower  develop- 


14  THE  PHYSIOLOGY  OF  TWINNING 

mental  rate  at  all  stages.  It  seems  obvious,  however, 
that  though  foreign  sperm  less  completely  stimulates 
the  egg  than  own  sperm,  yet  some  stimulus  is  given, 
for  hybrid  embryos  develop  more  rapidly  and  more 
normally  than  do  parthenogenetic  embryos. 

C.  Twins  due  to  crowding.—  Varying  percentages  of 
twins  of  an  even  more  advanced  type  were  frequently 
found  in  control  batches  of  eggs  that  had  been  fertilized 
by  own  sperm,  but  had  been  allowed  to  remain  in  much 
too  large  numbers  in  relatively  small  containers.  Some 
of  the  most  advanced  twin  larvae  were  discovered 
under  these  conditions,  and  such  twin  larvae,  when 
transferred  to  new  quarters,  where  they  had  plenty  of 
room  and  oxygen,  developed  farther  than  any  other 
twin  larvae  observed.  In  some  cases  they  practically 
recovered  the  single  condition  through  the  absorption  of 
the  smaller  twin  by  the  larger.  It  seems  obvious  that 
these  larvae  in  the  crowded  cultures  had  suffered  a 
period  of  growth  depression  due  to  the  low  oxygen  and 
high  C02  content  of  the  sea  water.  It  was  my  intention 
to  perform  some  experiments  involving  lack  of  oxygen, 
but  the  work  was  interrupted  before  there  was  oppor- 
tunity to  do  so. 

Retarded  development,  the  primary  cause  of  twinning. — 
In  all  three  of  these  methods  of  twinning  the  common 
factor  is  undoubtedly  a  retardation,  more  or  less  severe, 
of  the  normal  rate  of  development.  This  was  not  at  all 
an  unlooked-for  result;  quite  the  contrary.  One  who, 
like  the  writer,  had  for  years  realized  the  fact  that  most, 
if  not  all  developmental  anomalies,  are,  in  last  analysis, 
the  result  of  interferences  with  the  normal  developmental 
rate  of  organisms,  and  who  had  offered  a  theory  of  arma- 


EXPERIMENTAL  PRODUCTION  OF  TWINS         15 

dillo  twinning  based  on  the  idea  that  twinning  was  the 
direct  result  of  interrupted  development,  would  natu- 
rally be  on  the  lookout  for  just  this  type  of  result. 

THREE   TYPES   OF   ONE-EGG   TWINS 

From  the  point  of  view  of  their  mode  of  origin  three 
distinct  types  of  twins  were  readily  distinguishable:  (1) 
dwarf  larvae  resulting  from  the  physiological  isolation, 
followed  by  physical  isolation,  of  the  cleavage  products 
of  separate  blastomeres;  (2)  double,  triple,  or  multiple 
individuals  resulting  from  the  formation  of  two  or  more 
archentera;  (3)  " two-headed"  larvae  resulting  from  a 
dichotomy  of  the  anterior  end  of  the  archenteron. 

In  addition  to  these  three  primary  types  certain 
secondary  types  appeared  as  the  result  of  fusion  of 
adjacent  archentera  to  form  individuals  with  a  A -shaped 
archenteron,  single  in  front  and  divided  behind  (Fig.  5) . 
Each  of  these  types  will  be  discussed  separately. 

1.  Dwarf  larvae. — -These  larvae  always  appeared  in 
parthenogenetic  cultures.  When  the  very  much  belated 
process  of  cleavage  begins,  it  proceeds  with  so  little 
energy  that  many  eggs  start  the  first  cleavage  and  lose 
their  developmental  momentum  before  the  two  daughter- 
cells  are  fully  separated.  In  others  the  first  cleavage 
completes  itself,  but  the  second  cleavage  takes  place  in 
only  one  of  the  blastomeres.  The  belated  blastomere 
either  remains  permanently  undivided  or  else  resumes 
division  at  a  slower  rate  than  the  first.  There  is  obvious 
physiological  discoordination  between  the  two  sets  ol 
cells  thus  produced.  Each  produces  a  blastula,  oiu-  less 
normal  than  the  other,  and  before  long  they  rupture 
the  membrane  and  become  swimming,  half-sized  larvae. 


16  THE  PHYSIOLOGY  OF  TWINNING 

Neither  one  is  fully  normal,  but  both  at  least  begin 
gastrulation  and  some  of  them  form  fairly  normal 
gastrulae.  Never,  however,  do  these  dwarfs  attain  a 
condition  that  approximates  that  of  a  bipennaria  larva. 
They  all  stop  at  the  gastrula  stage.  We  may  denominate 
this  mode  of  origin  of  twins  physiological  blastotomy; 
for  it  is  the  result  of  the  physiological  isolation  of  the 
first  two  blastomeres,  so  that  they  act  as  though  they 
were  independent  eggs.  Such  a  condition  is  equivalent 
to  the  occurrence  of  two  separate  blastoderms  on  a 
fish  or  a  chick  egg. 

2.  Twins  with  two  or  more  archentera.- — -Much  the 
commonest  mode  of  twinning  in  the  starfish  is  one  in 
which  two  or  more  points  of  invagination  occur,  resulting 
in  two  or  more  archentera.  Many  varieties  of  this 
type  of  twin  occur.  A  very  common  type  is  that  in 
which  the  original  archenteron  seems  to  persist  and  a 
secondary  one  (often  two  or  three)  arise  at  the  opposite 
or  apical  end  of  the  larva.  Such  a  larva  may  sub- 
sequently rid  itself  of  these  accessory  archentera  by 
closing  their  blastopores  and  pinching  off  the  small 
archentera  so  as  to  leave  one  or  more  small  internal 
vesicles  or  cysts  that  remain  attached  to  the  body  wall 
for  some  time,  only  to  be  ultimately  resorbed.  Some 
biaxiate  larvae,  however,  such  as  that  shown  in  Figures  i 
and  2,  have  the  secondary  archenteron  nearly  as  large  as 
the  primary  and  the  animal  exists  as  a  biaxiate  bipen- 
naria for  a  long  time.  In  swimming,  such  a  larva  moves 
in  a  direction  determined  by  the  position  of  the  primary 
blastopore,  i.e.,  with  this  structure  posteriorly  directed. 
The  anterior  component  of  this  twin  seems  to  be  under 
the  dominance  of  the  posterior  or  primary  component. 


EXPERIMENTAL  PRODUCTION  OF  TWINS 


17 


Another  common  type  of  twin  larva  is  one  in  which 
there  seems  to  have  been  a  partial  obliteration  of  the 
primary  symmetry  relations  and  two  secondary  equiv- 
alent symmetries  result.  In  such  larvae  both  archen- 
tera  develop  symmetrically  and  are  quite  equivalent, 
so    that    it   wrould    not   be   possible   to   call    one    the 


Figs.  1-3. — Outline  drawings  of  three  types  of  twin  larvae  of  the 
starfish  Patiria  miniata,  showing  plural  archentera.  Fig.  1 .  an  advanced 
bipennaria  with  primary  archenteron  at  the  original  basal  pole  and  a  well- 
developed  secondary  archenteron  at  the  site  of  the  original  apical  poK  . 
Fig.  2,  a  similar  bipennaria  with  two  supernumerary  archentera,  b 
secondary  and  a  tertiary.  Fig.  3,  a  true  twin  larva  with  two  sym- 
metrically placed  archentera,  neither  of  which  is  the  primary  one. 
(From  Newman.) 


i8 


THE  PHYSIOLOGY  OF  TWINNING 


primary  and  the  other  the  secondary  component.  Such 
twin  larvae  develop  symmetrically  throughout  their 
lives  and  reach  fairly  advanced  stages.  Typical  examples 
of  the  relations  of  the  two  components  are  shown  in 


Figs.  4-6. — Three  very  common  types  of  symmetrical  twin  larvae 
of  Patiria  miniata.  Scores  of  such  twins  were  found  in  almost  every 
batch  of  larvae  which  were  in  any  way  retarded  in  development. 
(Original.) 

Figures  3,  4,  5,  6,  7.  When  in  larvae  of  this  sort  one 
of  the  archentera  is  originally  smaller  or  secondarily 
comes  to  be  less  active  in  its  growth  than  the  other,  it 
quite  commonly  happens  that  the  smaller  one  comes  into 
contact  with  the  larger,  fuses  with  it  at  its  anterior 
end,  and,  after  closing  its  blastopore,  becomes  first  a 
pouch  of  the  larger  archenteron  and  then  merely  a 
thickening  in  the  wall  of  the  latter.     This  is  a  very 


EXPERIMENTAL  PRODUCTION  OF  TWINS 


19 


striking  example  of  the  way  in  which  a  stronger  twin 
may  dominate  over  and  ultimately  rid  itself  of  a  weaker 
twin  so  as  to  come  to  be  practically  a  normal  individual. 
Occasionally  also  two  quite  equivalent  paired  archentera 
grow  toward  one  another  at  such  an  angle  that  they  meet 


Figs.  7,  8. — Two  advanced  larvae  of  Patiria.  Fig.  7,  a  symmetrical 
twin  with  the  two  separate  alimentary  tracts  considerably  differentiated 
and  exhibiting  striking  mirror-image  symmetry.  Fig.  8,  a  twin  of  the 
same  age  with  the  anterior  ends  of  the  two  alimentary  tracts  fused  to 
form  a  common  pharynx.     (From  Newman.) 

in  the  middle  of  the  cleavage  cavity  and  fuse  at  their 
anterior  ends.  Such  larvae  come  to  have  one  anterior 
and  two  posterior  archentera  and  two  blastopores. 
In  several  cases  such  larvae  as  this  (Fig.  8)  went  so 
far  as  to  differentiate  two  stomach  enlargements,  two 
oesophagi  and  two  intestines,  communicating  with  a 
common  pharynx. 

3.  Twins  with  bifurcated  archcntcron. — A  relatively 
rare  type  of  retarded  larva  undergoes  dichotomy  of  the 
anterior  end  of  the  archenteron,  which  results  in  a  sort 
of    "double-headed"    condition.     This    phenomonen    is 


20 


THE  PHYSIOLOGY  OF  TWINNING 


discussed  in  a  subsequent  section  under  the  caption  An 
adidymi  among  starfishes  (pp.  23,  24). 

The  mode  of  origin  of  these  twins  resulting  from 
plural  gastrulation  processes  is  of  considerable  interest. 
One  can  easily  trace  the  twinning  back  to  the  blastula 


// 


13 


10 


12 


Figs.  9-14.— Types  of  blastulae  with  appropriate  gastrulae  of  Patiria. 
Fig.  9,  a  typical  blastula,  with  the  typical  gastrula  (Fig.  10)  that  arises 
from  it.  Fig.  11,  a  blastula  that  has  entirely  lost  its  polarity,  with  the 
very  abnormal  multiple  gastrula  (Fig.  12)  that  arises  from  it.  Fig.  13^ 
a  bipolar  blastula,  the  original  polarity  of  which  has  been  only  partially 
lost,  with  the  bipolar  gastrula  (Fig.  14)  that  arises  from  it.  (From 
Newman.) 

stage.  A  normal  starfish  blastula  has  a  very  definite 
primary  polarity.  As  shown  in  Figure  9,  the  blastula 
wall  is  much  thicker  on  one  side  than  the  other.  The 
cells  of  the  thicker  region  are  loaded  with  yolk  granules 


EXPERIMENTAL  PRODUCTION  OF  TWINS         21 

and  those  at  the  thinner  pole  are  free  from  yolk.  Obvi- 
ously then  we  have  a  well-defined  animal  and  vegetal 
pole,  and  gastrulation  or  archenteron  formation  is  an  in- 
growth or  invagination  of  the  vegetal  pole  (Fig.  10).  In 
the  case  of  parthenogenetic  blastulae,  however,  numerous 
deviations  from  the  normal  conditions  are  observed: 

a)  Some  blastulae  remain  nearly  solid,  showing 
scarcely  any  cleavage  cavity.  Such  blastulae  never 
gastrulate  at  all. 

b)  Other  blastulae  (Fig.  11)  are  without  any  polarity. 
The  cleavage  cavity  is  large  but  the  yoLk  material  is 
evenly  distributed  among  all  of  the  cells.  Such  blastulae 
usually  undergo  multiple  gastrulation,  the  surface  in- 
vaginating  intricately  as  in  Figure  12. 

c)  Other  blastulae,  instead  of  having  one  thickened, 
yolk-laden  region  of  the  blastoderm,  have  two  or  more 
such  regions  (Fig.  13).  Such  bipolar  and  tripolar 
blastulae  invaginate  at  two  or  three  places  to  make  the 
types  of  larvae  in  which  there  are  two  or  three  archen- 
tera.  A  typical  bipolar  gastrula  is  shown  in  Figure  14. 
If  the  original  polarity  is  retained  to  some  extent  and 
only  a  relatively  small,  thickened,  yolk-laden  area 
appears  at  the  opposite  pole,  we  get  gastrulae  and  bipen- 
nariae  of  a  bipolar  sort  with  a  supernumerary  archenteron 
at  the  original  apical  end.  Sometimes  the  thickening 
at  the  apical  end  fails  to  invaginate  and  looks  somewhat 
like  an  "apical  plate."  It  has,  in  fact,  been  so  inter- 
preted (see  Heath,  1906). 

d)  Sometimes  the  thickened  basal  area  becomes  much 
broader  than  normal  and  has  a  thinned-out  region  in  the 
middle,  as  though  a  sort  of  fission  of  the  vegetal  pole  had 
occurred.     Such  blastulae  produce  true  identical  twin 


22  THE  PHYSIOLOGY  OF  TWINNING 

gastrulae  with  equivalent  and  symmetrical  archentera. 
If  the  fission  is  somewhat  unilateral  or  asymmetrical, 
as  it  often  is,  twin  archentera  of  different  sizes  occur  and 
the  larger  often  becomes  dominant  over  the  smaller,  deter- 
mines the  axis  of  the  embryo,  and  absorbs  the  smaller. 

TWINNING  A   RESULT   OF   A   LOSS   OF   POLARITY 

Physiologically  considered,  what  happens  in  all  these 
cases  is  this:  A  lowering  of  the  developmental  or  meta- 
bolic rate  of  the  embryo,  either  before  or  during  cleavage, 
has  to  a  more  or  less  complete  extent  obliterated  the 
original  polarity,  which  has  been  shown  to  depend  on  a 
gradient  of  oxidative  and  other  activities  running  from 
the  animal  or  apical  pole  to  the  vegetal  or  basal  pole 
of  the  egg  or  embryo.  The  rate  of  metabolism  of  the 
whole  is  lowered  to  such  an  extent  that  in  extreme 
cases  the  whole  gradient  is  obliterated,  with  the  result 
that  no  point  is  distinctly  apical  to  any  other;  so  that 
any  point  may  acquire  independence  and  start  to 
invaginate  if  stimulated  so  to  do.  If  the  original 
gradient  is  only  partially  lost  certain  secondary  basal 
regions  may  become  isolated  and  begin  independent 
invagination.  If  the  original  basal  region  merely  under- 
goes fission  we  get  twin  or  equivalent  archentera. 

I  look  upon  the  process  of  normal  gastrulation  as  a 
condition  much  like  the  formation  of  a  new  zooid  in  a 
planarian.  The  ingrowth  of  the  archenteron  at  the 
point  most  distant  from  the  original  apical  end  of  the 
embryo  is  due  to  a  physiological  isolation  of  a  new 
actively  growing  region  that  is  highly  susceptible  to 
growth  disturbances.  It  is  the  ingrowth  of  the  archen- 
teron that  establishes  the  new  axis  of  symmetry  and, 


EXPERIMENTAL  PRODUCTION  OF  TWIN-  23 

indeed,  the  whole  organization  of  the  embryo.  The 
old  egg  symmetry  largely  passes  away  and  a  new  sym- 
metry of  the  embryo  and  larva  takes  its  place.  In  the 
echinoderms,  of  course,  even  this  new  antero-posterior 
axis  and  bilateral  symmetry  are  largely  done  away  with 
when  the  larva  undergoes  metamorphosis  into  the 
radially  symmetrical  adult.  This,  however,  does  not 
concern  us  in  the  present  connection. 

SECONDARY   PHASES    OF    GASTRULATION 

Although,  as  has  been  said,  the  initial  steps  of 
gastrulation  involve  an  inpushing  of  a  relatively  basal 
region  characterized  by  low  metabolic  rate,  a  new 
apical  point  soon  arises  at  the  distal  or  ingrowing  end 
of  the  archenteron.  Such  a  region  becomes,  in  a  sense, 
the  head  end  of  the  larva  and  is  the  most  actively 
growing  and  differentiating  region  of  the  latter.  In  a 
few  cultures  of  Patina  there  occurred  larvae  reminding 
one  strongly  of  common  types  of  double-headed  human 
and  fish  monsters  known  technically  as  anadidymi, 
individuals  divided  anteriorly  and  united  posteriorly. 

ANADIDYMI   AMONG    STARFISH   LARVAE 

These  larvae  occurred  in  cultures  that  had  been 
normally  fertilized  and  in  which  the  great  majority  of 
individuals  were  quite  typical.  What  had  been  the  cause 
of  their  occurrence  was  not  clear.  These  larvae,  how- 
ever, were  obviously  retarded  forms,  for  they  were 
considerably  smaller  and  less  active  than  normal  larvae. 
They  seem  to  have  developed  normally  until  they  had 
reached  a  late  gastrula  stage,  a  stage  when  it  may  be 
said  that  the  axis  of  symmetry  had  been  well  established. 
Then,  through  a  dichotomy  of  the  anterior  end  of  the 


24 


THE  PHYSIOLOGY  OF  TWINNING 


archenteron,  they  became  distinctly  " two-headed."  In 
some  of  the  individuals  (Figs.  15,  16)  the  median  parts 
were  quite  incompletely  separated  and  inner  struc- 
tures remained  united  in  the  middle.  In  others,  as  in 
Figure  17,  the  two  " heads"  became  separate  and  only 
the  posterior  part  of  the  archenteron  remained  in  com- 


Figs.  15-17. — Patiria  larvae  in  which  the  anterior  end  of  the  archen- 
teron has  undergone  dichotomy.  Fig.  15,  a  larva  with  only  a  slight 
degree  of  dichotomy.  Fig.  16,  a  larva  with  a  moderate  degree  of  dichot- 
omy. Fig.  17,  a  larva  with  complete  dichotomy  of  the  anterior  end  of 
the  archenteron.     (From  Newman.) 

mon.  For  some  unknown  reason,  larvae  of  this  sort 
fail  to  advance  much  farther  than  the  stages  shown,  and 
I  was  unable  to  discover  the  further  consequences  of 
such  a  process  of  twinning. 

SECONDARY  FUSIONS  AND  THEIR  CONSEQUENCES 

As  has  already  been  said,  fusions  frequently  occur 
between  archentera  that  arise  closely  adjacent  to  each 
other.  I  have  rarely  seen  a  case  of  fusion  between  two 
archentera  that  had  arisen  from  distinctly  separate 
basal  areas.  Only  when  the  two  archentera  are  the  result 
of  the  fission  of  a  single  basal  area  do  they  exhibit  a 
strong  tendency  to  fuse.  If  the  two  are  of  equal  size,  i.e., 
are  identical  twins,  they  often  grow  together  and  fuse 


PROPERTY  LIBRARY 


EXPERIMENTAL  PRODUCTION  OF  TWINS         25 

by  the  anterior  ends  of  their  archentera,  making  a  type 
of  embryo  with  one  anterior  end  and  a  divided  posterior 
like  the  type  of  double  monster  known  technically  as 
katadidymus  (Fig.  5). 

THE   INFLUENCES    OF   ONE   TWIN   UPON  ANOTHER 

One  of  the  most  significant  features  of  twinning  in 
the  starfish  has  to  do  with  the  apparent  control  one 
twin  component  has  over  another.  When,  as  in  cases 
such  as  those  shown  in  Figures  1  and  2,  one  of  the 
components  is  distinctly  the  primary  individual  and 
the  other  is  secondary,  only  the  primary  archenteron 
is  able  to  differentiate  in  normal  fashion.  There  are 
numerous  instances  in  which  the  primary  archenteron 
produces  its  coelomic  pouches  and  breaks  through  a 
mouth,  while  the  secondary  archenteron  either  closes  the 
blastopore  and  becomes  a  cyst  or  else  remains  in  an 
undifferentiated  condition.  In  such  cases  it  seems  that 
the  primary  component  must  in  some  way  exercise  an 
inhibiting  influence  upon  the  secondary  component. 
Just  what  may  be  the  mechanism  of  such  an  inhibition 
we  do  not  know  for  sure,  but  it  seems  highly  probable 
that  it  is  a  phenomenon  involving  the  exercise  of  domi- 
nance and  subordination  through  the  gradient.  It  is 
probable  that  the  gradient  of  the  primary  individual, 
on  meeting  that  of  the  secondary,  tends  to  overwhelm 
the  latter  and  reverse  its  direction,  thus  making  it  a 
subordinate  part  of  the  primary  gradient.  That  this  is 
more  than  a  mere  conjecture  is  evidenced  by  the  fact 
that  in  a  specimen  like  that  shown  in  Figure  1  the  din 
tion  of  ciliary  beat  in  the  secondary  component  is  at 
least  mainly  away  from  its  own  blastopore,  instead  <>t 


26 


THE  PHYSIOLOGY  OF  TWINNING 


toward  it  as  it  would  be  if  this  individual  had  control 
of  its  own  gradient. 

An  even  more  complete  domination  of  a  weaker 
individual  by  a  stronger  occurs  when  one  of  a  pair  of 
archentera  arising  from  one  continuous  basal  area  is 
distinctly    smaller    than    the    other.     I    have    watched 


18 

Figs.  18-21. — Four  outline  figures  of  a  single  Patiria  larva,  drawn 
on  four  successive  days,  to  show  the  way  in  which  a  twinned  larva  not 
infrequently  returns  to  an  almost  normal  single  or  untwinned  condition. 
The  smaller  twin  archenteron  becomes  a  sort  of  parasite  upon  the  side 
of  the  larger.     See  text  for  details.     (Original.) 

from  day  to  day  all  of  the  stages  of  such  a  process.  The 
series  of  drawings  (Figs.  18,  19,  20,  21)  were  made 
from  a  single  individual  at  intervals  of  about  twenty-four 
hours.  It  is  remarkable  how  nearly  like  a  single  nor- 
mal individual  such  a  twin  can  become  after  absorbing 
its  weaker  brother,  though  neither  one  of  them  is  truly 
primary  in  the  sense  that  it  represents  the  original 
individual,  nor  secondary  in  the  sense  that  one  is  a 
bud  from  the  other.  This  rinding  has  a  very  definite 
bearing  on  theories  of  the  origin  of  double  monstrosities 
in  the  higher  animals,  especially  in  vertebrates.  There 
is  frequently  a  marked  difference  in  the  size  and  degree 
of  normality  in  the  two  components  in  double  monsters 
and  it  frequently  happens  that,  as  in  the  starfish  cases 


EXPERIMENTAL  PRODUCTION  OF  TWINS         27 

just  described,  the  larger  more  or  less  completely  absorbs 
or  grows  over  the  smaller.  The  larger  component  has 
come  to  be  called  the  "autosite"  and  the  smaller,  the 
"parasite."  The  genetic  relationship  between  the  two  has 
been  discussed  by  Stockard,  who  considers  the  parasite 
component  as  a  lateral  bud  derived  from  the  autosite 
and  kept  in  subordination  to  the  latter  much  as  a  lateral 
plant  bud  is  inhibited  by  the  growth  of  the  main  growing- 
point.  That  such  an  explanation  as  this  is  inapplicable 
to  the  condition  in  the  starfish  need  hardly  be  stated. 

CONCLUSIONS 

The  reader  will  now  have  become  aware  that  in  the 
simple  development  of  the  starfish  there  appear,  dia- 
grammatically  almost,  practically  all  of  the  various 
phenomena  that  are  associated  with  one-egg  twinning. 
We  have  been  able  to  observe  all  of  the  stages  of  develop- 
ment and  to  see  many  of  the  events  of  the  twinning 
process.  We  know,  moreover,  to  a  considerable  extent 
the  causes  of  the  various  twinning  processes  observed. 
If  we  are  to  understand  the  more  intricate  twinning 
phenomena  in  the  armadillo,  in  man,  in  the  fishes,  and 
in  various  other  animals  or  even  plants,  it  seems  clear 
that  we  shall  have  to  refer  these  conditions  back  to  th«>-c 
of  the  starfish  as  a  sort  of  norm,  and  it  is  my  present 
opinion  that  the  starfish  situation  throws  much  li,urht  on 
the  whole  problem  of  the  physiology  of  twinning. 
I  therefore  scarcely  need  to  offer  an  apology  for  uri\inur 
first  place  in  the  present  volume  to  the  lowly  starfish 
and  for  relegating  to  less  prominent  positions  tho 
phases  of  twinning  that  are  more  familiar  and  that  are 
more  imbued  with  human  interest. 


CHAPTER  III 

TWINNING  IN  EARTHWORMS  AND  THEIR  ALLIES 

One  of  the  earliest  studies  of  one-egg  twinning  was 
that  of  Kleinenberg  (1878)  on  the  earthworm  Lumbricus 
trapezoides.  The  condition  described  by  him  is  of  great 
interest  in  our  study  of  twinning,  as  it  was  perhaps  the 
first  described  case  of  twinning  in  which  it  was  certain 
that  only  one  egg  was  involved. 

Kleinenberg  found  that  of  the  three  to  eight  eggs 
found  in  a  cocoon  or  capsule  only  one,  or  occasionally 
two  or  three,  undergoes  development,  the  others,  which 
he  believes  to  have  been  unfertilized,  undergoing  com- 
plete disintegration  within  the  capsule.  Although  the 
author  does  not  lay  any  stress  upon  this  condition,  I 
would  like  just  here  to  point  out  that  the  environment 
of  the  developing  egg  or  eggs  in  a  capsule,  fouled  by  the 
decay  of  several  other  eggs,  is  not  at  all  likely  to  result 
in  normal  development. 

The  embryonic  history  of  the  eggs  that  survive  is  as 
follows :  Cleavage  is  apparently  normal  up  to  a  blastula 
stage,  consisting  of  a  thin-walled,  bladder-like  vesicle. 
Gastrulation  apparently  occurs  through  the  more  rapid 
growth  of  the  cells  at  one  end  of  the  vesicle.  At  the  pole 
of  greater  thickness  two  large  cells,  the  primary  mesoblast 
cells,  become  pushed  in  and  are  overgrown  by  small 
surface  cells.  From  these  two  mesoblast  cells  there  arise 
by  proliferation  two  rows  of  cell  masses  destined  normally 
to  be  the  right  and  left  endodermal  and  mesodermal 

28 


TWINNING  IN  EARTHWORMS  29 

structures  of  a  single  animal.  While  this  process  of 
mesoblast  elongation  is  taking  place  a  longitudinal  con- 
striction occurs,  beginning  at  about  the  middle  and 
running  both  forward  and  backward  until  it  surrounds 
the  embryo.  This  furrow  cuts  more  or  less  deeply 
through  the  embryo,  dividing  the  right  half  from  the 
left.  In  the  majority  of  individuals  the  two  half-bodies 
remain  connected  in  the  middle  region  by  only  a  few 
large  ectodermal  cells.  Each  half-embryo,  when  thus 
isolated,  develops  into  an  entire  embryo.  Here  we  have 
a  true  case  of  duplicate  twins  derived  from  the  right  and 
left  halves  of  a  single  embryo.  When  the  slightly 
joined  twin  worms  become  active  they  complete  their 
physical  separation  by  means  of  a  series  of  rotations 
which  twist  and  finally  break  off  the  uniting  cellular 
cord. 

Double  monsters  result  when  the  connection  is  too 
thick  to  admit  of  twisting  apart.  Sometimes  the 
united  region  is  of  considerable  extent,  but  the  union 
involves  only  the  external  epithelium  and  not  at  all  the 
internal  structures.  Sometimes  the  two  components  of 
these  double  monsters  are  of  very  unequal  size,  one  of 
them  being  the  equivalent  of  an  autosite  and  the  other 
of  a  parasite,  if  we  may  use  the  terminology  employed 
for  similar  situations  in  vertebrate  conjoined  twins. 
One  interesting  feature  of  Kleinenberg's  work  is  that, 
in  the  species  studied  by  him,  twinning  appears  to  be 
almost  as  regular  and  specific  a  process  as  it  is  in  the 
armadillos.  In  all  of  his  numerous  cases  only  a  very 
few  instances  occurred  in  which  but  a  single  worm 
emerged  from  an  egg  capsule,  and  even  these  single 
worms  were  probably  survivors  of  twins,  for  rudiments 


30  THE  PHYSIOLOGY  OF  TWINNING 

of   a   degenerate   or   absorbed   twin  were  usually  dis- 
tinguishable. 

Vejdovsky  (1882-92)  a  few  years  later  made  an 
extensive  study  of  twinning  in  three  other  species  of 
earthworms,  Lumbricus  terrestris,  Allolobophora  foetida, 
and  Allolobophora  trapezoides.  Only  two  twinned  speci- 
mens were  found  in  the  first  species,  two  in  the  second, 
and  large  numbers  in  the  third.  He  found  a  great 
variety  of  conditions  which  may  be  summarized  as 
follows:  (1)  One  case  in  which  the  components  were 
united  on  the  ventral  side  along  the  whole  length  of  the 
body.  (2)  Cases  in  which  union  was  along  the  dorsal 
side  (frequent).  (3)  Cases  in  which  union  was  end  to 
end  (rare).  In  all  of  these  types,  except  in  those  where 
the  union  was  throughout  the  entire  length,  there  occurred 
cases  of  more  or  less  marked  size  inequality  between  the 
two  components.  Vejdovsky  states  that  completely  sep- 
arated twins  occur  with  extreme  rarity  in  the  species 
studied  by  him. 

Weber  (191 7)  made  a  study  of  double  monsters  in 
the  earthworm  Helodrilus  caliginosus  trapezoides.  In 
this  species  there  occur  many  completely  separate  twins 
as  well  as  large  numbers  of  conjoined  twins.  It  is  also 
important  to  note  that  in  nearly  a  third  of  the  cases 
observed  an  egg  gave  rise  to  but  a  single  individual  and 
that  a  few  eggs  produced  quadruplets.  The  majority 
formed  double  monsters.  Since  some  significance 
attaches  to  the  manner  in  which  the  components  of 
these  conjoined  twins  were  united,  certain  details  of 
Miss  Weber's  observations  must  be  given.  She  classifies 
the  double  monsters  according  to  their  mode  of  union 
as  follows:    (1)   Those  in  which  the  union  is  dorsal; 


TWINNING  IN  EARTHWORMS  31 

(a)  those  in  which  the  union  extends  to  the  alimentary 
tracts;  (b)  those  in  which  alimentary  tracts  are  not 
united.  (2)  Those  in  which  the  union  is  latero-dorsal 
(one  example).  (3)  Those  in  which  union  is  end  to  end 
and  in  which  the  cerebral  ganglia  are  double  and  on  op- 
posite sides  of  the  digestive  tract.  The  union  is  dorsal,  as 
the  nerve  cords  are  on  the  free  (opposite)  sides  of  the 
united  region.  (4)  Those  in  which  union  is  lateral,  the 
components  lying  side  by  side  with  the  mouth  of  both  on 
the  same  side.  (5)  Those  in  which  the  two  components 
are  extremely  unequal  in  size. 

It  is  a  significant  fact  that  no  cases  were  found  in 
which  the  union  is  ventral.  This  leads  Miss  Weber  to 
believe  that  the  single  instance  cited  by  Vejdovsky  is  a 
misinterpretation  and  that  what  he  had  was  a  case  of 
dorsal  union,  the  confusion  being  due  to  the  close 
approximation  of  ganglia  from  opposite  sides  which  lay 
in  the  semblance  of  a  paired  arrangement. 

As  to  the  causes  of  twinning  in  the  earthworms 
Kleinenberg  makes  the  unsupported  suggestion  that  the 
doubling  is  due  to  the  entrance  of  two  sperms  into  a 
single  egg.  Such  a  suggestion  is  nowadays  quite  unten- 
able. Vejdovsky  is  inclined  to  adopt  a  physiological 
explanation  suggesting  the  possibility  that  twinning 
may  be  due  to  environmental  factors,  such  as  tempera- 
ture, moisture,  or  exposure  to  air. 

Korchelt  (1904)  described  and  figured  (Fig.  22)  an 
interesting  double  monster  of  the  earthworm  Allobophora 
siibrubicunda.  This  is  a  very  typical  case-  and  will  serve 
as  a  type  for  the  whole  group.  This  author  also  suc- 
ceeded in  producing  large  numbers  of  double-headed  and 
double-tailed  worms   by  regeneration    methods.     After 


32 


THE  PHYSIOLOGY  OF  TWINNING 


cutting  out  a  section  of  worm  from  near  the  middle, 
the  anterior  and  posterior  ends  regenerate  very  frequently 
into  double  or  even  triple  heads  or  tails.  Doubling  or 
twinning  in  the  course  of  regeneration  after  cutting  the 

body  or  appendages  of 
animals  or  plants  is  an 
extremely  common  phe- 
nomenon. I  consider 
that  the  slowness  with 
which  early  regrowth 
occurs  allows  the  axiate 
organization  of  the  tissue 
to  be  lost,  and  new  grow- 
ing regions  arise  some- 
times at  two  or  more 
places,  thus  resulting  in 
duplicity  or  triplicity. 

TWINNING  IN  THE  MICRO- 
DRILOUS  OLIGOCHAETES 

The  very  recent  ac- 
count of  Welch  (192 1), 
although  not  illustrated, 
affords  data  of  very  con- 
siderable significance 
with  reference  to  the 
phenomenon  which  he 
calls  bifurcation.  He 
made  a  study  of  the 
contents  of  over  500  co- 
coons of  Tubifex  tubifex, 
which   were    obtained 


Fig.  22. — Typical  twin  earthworm 
of  the  species  Allolobophora  snbrubi- 
cunda,  slightly  bifid  at  the  anterior 
end  and  extensively  bifid  at  the  pos- 
terior end.  In  other  larvae  the  condi- 
tion may  be  reversed  or  else  one  end 
may  be  single  and  the  other  bifid. 
(After  Korschelt.) 


TWINNING  IN  EARTHWORMS  33 

from  pond  and  river  mud  by  means  of  a  sieve.  Each 
cocoon  contains  from  one  to  fourteen  eggs  or  embryos, 
the  average  being  about  nine.  The  only  avenue  of 
escape  from  the  cocoon  is  through  the  two  necks  of 
the  latter.  When  temporary  plugs  in  these  necks  are 
removed  the  apertures  are  just  wide  enough  for  a  normal 
young  worm  to  emerge. 

About  20  per  cent  of  the  cocoons  examined  showed 
one  or  more  double-headed  or  double-tailed  worms, 
only  one,  two,  or  three  bifid  worms  being  present  in 
each.  The  various  types  of  duplicity  are  classified  by 
Welch  as  follows: 

A.  Either  anterior  or  posterior  extremity  bifid 

I.  Bifurcation  simple 

a)  Branches  equal 

b)  Branches  unequal 

II.  Bifurcation  compound 

a)  Plane  of  bifurcation 

(1)  Secondary  bifurcation  in  same  plane  as  primary 

(2)  Secondary  bifurcation  at  right  angles  to  primary 

b)  Equality  of  bifurcation 

(1)  Parts  of  primary  bifurcation  equal;  secondary  equal 
or  unequal 

(2)  Parts  of  primary  bifurcation  unequal;    secondary 
equal  or  unequal 

B.  Both  anterior  and  posterior  extremities  bifid 

I.  Primary  bifurcations  in  same  plane 

II.  Primary  bifurcations  in  different  planes 

III.  Either  or  both  bifurcations  compound 

It  was  further  brought  out  that  anterior  bifurcations 
with  normal  posterior  end  are  about  twice  as  numerous 
as  those  with  posterior  bifurcation  and  normal  anterior 
end.     Furthermore    those   with    both    ends    bifurcated 


34  THE  PHYSIOLOGY  OF  TWINNING 

are  about  as  numerous  as  those  bifid  at  the  anterior  end 

alone. 

It  is  very  rare  for  bifid  individuals  to  be  able  to 
emerge  from  the  cocoon,  only  ten  young  worms  out  of 
4,000  that  had  emerged  being  bifid.  The  forked  ends 
tend  to  inhibit  emergence  because  the  opening  of  the 
cocoon  is  only  large  enough  for  a  normal  worm.  Those 
with  deep  bifurcations  both  anteriorly  and  posteriorly 
could  not  possibly  escape.  The  only  chance  of  successful 
emergence  seems  to  be  to  start  out  with  the  single  end 
first.  A  few  escaped  that  had  a  deep  bifurcation  at  one 
end  and  a  very  slight  one  at  the  other.  Thus  the  hazards 
of  one-egg  twinning  seem  to  be  even  greater  for  worms 
than  will  be  shown  to  be  the  case  in  man.  If  there 
were  in  these  worms  an  inherited  tendency  to  twin,  it 
could  not  successfully  be  passed  on,  for  the  survivors 
are  far  too  infrequent  to  admit  of  such  a  character 
being  transmitted  in  so  large  a  percentage  of  individuals 
as  actually  occur  in  cocoons.  The  only  alternative 
conclusion,  then,  is  that  the  condition  is  due  to  environ- 
mental factors  such  as  those  already  discussed. 

THE   MODES    OF   TWINNING  IN   THE    OLIGOCHAETES 

The  work  of  Hyman  and  others  has  shown  that  in 
annelids,  and  especially  in  oligochaetes,  the  head  end  is  at 
first  the  only  apical  end  and  it  is  highly  susceptible  to 
growth-inhibiting  agents.  Relatively  early  in  develop- 
ment, however,  the  posterior  end  of  the  embryo  becomes 
a  secondary  apical  point  with  a  forward-directed  gradi- 
ent. Thus  these  worms  have  a  double  axiate  organi- 
zation with  two  highly  susceptible  regions.  It  is  very 
significant  that  bilateral  doubling  (true  twinning)  occurs 


TWINNING  IN  EARTHWORMS  35 

most  frequently  and  is  most  complete  in  the  two  apical 
regions  of  the  double  gradient  and  that  the  part  of  the 
body  that  most  frequently  fails  to  undergo  twinning  is 
that  region  which  represents  the  common  basal  region, 
or  region  of  lowest  metabolic  rate,  of  the  two  gradients. 
It  is  also  important  for  us  to  note  that  such  bilateral 
organisms  as  the  worms  show  almost  exactly  the  same 
types  of  double  monstrosities  as  have  been  described 
for  the  vertebrates.  The  explanation  of  the  fact  that 
heads  and  tails  are  double  and  the  middle  of  the  body 
single  is  that  the  free  ends  have  undergone  bifurcation 
or  longitudinal  fission.  There  is  not  even  the  shadow 
of  suggestion  that  the  double  monsters  arose  as  products 
of  the  fusion  of  two  individuals  arising  from  separate 
embryonic  axes,  for  there  is  really  no  possibility  that 
such  a  thing  could  occur  in  these  worms.  The  fact 
that  all  sorts  of  double  monsters,  in  these  simple,  bilateral, 
metameric  organisms  actually  do  occur  by  dichotomy 
of  the  apical  ends  goes  far  to  support  our  theory  of  the 
origin  or  conjoined  twins  in  the  vertebrates.  It  should 
also  be  said  that  every  degree  of  bifurcation  is  present, 
from  a  very  slight  terminal  broadening  to  a  very  deep 
bifurcation  running  nearly  half  the  length  of  the  worm. 
This  situation  also  entirely  parallels  the  condition  seen 
in  the  vertebrates  and  showrs  that  individuals  vary 
greatly  in  their  responses  to  the  factors  that  induce 
twinning.  Another  situation  in  these  double  worms 
that  parallels  that  in  vertebrate  double  monsters  is 
that  the  portion  of  the  body  that  is  apparently  single 
is  in  reality  nearly  all  double.  Thus  there  are  usually 
two  complete  nerve  cords  in  the  united  part  of  the  body 
and  they  are  1800  apart;    which  means  that  the  ventral 


36  THE  PHYSIOLOGY  OF  TWINNING 

parts  are  separate  and  the  dorsal  united.  This  is  just 
the  opposite  of  what  we  find  in  the  vertebrates,  but  is 
precisely  what  we  would  expect  in  view  of  the  fact  that 
the  nervous  system  is  dorsal  in  the  vertebrates  and 
ventral  in  the  annelids. 

CAUSES    OF   TWINNING  IN   THE   OLIGOCHAETA 

The  method  of  reproduction  in  the  Oligochaeta  is 
the  well-known  one  of  cocoon  formation  which,  at  the 
risk  of  repeating  what  every  biologist  knows,  may  be 
stated  briefly  as  follows:  During  copulation  a  tough 
girdle  composed  of  hardened  slime  is  formed  about  the 
chtellum  of  each  worm.  After  the  pair  separates,  each 
slime  girdle  which  is  destined  to  be  a  cocoon  is  slowly 
worked  forward,  collecting  albumen  from  the  glands 
on  the  ventral  surface.  It  is  sloughed  off  over  the 
head,  passing  first  the  openings  of  the  oviducts  where 
it  receives  eggs,  then  that  of  the  seminal  receptacles 
where  it  receives  sperm.  The  two  free  ends  close  as 
though  with  a  drawstring  and  the  closed  cocoon  is 
dropped  on  the  ground  or  in  the  mud.  It  has  already 
been  pointed  out  that,  at  least  in  some  species,  some  of 
the  eggs  die  and  decay.  This  would  greatly  foul  the 
contents  of  the  cocoon.  Whether  some  eggs  die  or  not, 
the  living  eggs  must  develop  with  a  limited  supply  of 
oxygen.  It  is  this  condition,  I  believe;  that  is  at  the 
bottom  of  the  twinning  process.  Lack  of  oxygen 
probably  so  retards  early  development  at  a  time  when 
abundant  oxygen  is  demanded  that  the  bilateral  pri- 
mordia  become  physiologically  more  or  less  completely 
isolated.  Isolation  occurs  more  extensively  at  the  points 
of  highest  rate  of  metabolism,  the  anterior  and  posterior 


TWINNING  IN  EARTHWORMS  37 

ends  and  the  ventral  surface,  and  is  less  complete  at 
regions  of  lower  rate  of  metabolism,  the  middle  parts  of 
the  body  and  the  dorsal  side.  In  other  words,  inhibition 
strikes  the  apical  points  of  the  axis  of  polarity  and  the 
axis  of  symmetry,  just  as  our  general  theory  demands. 

The  type  of  twinning  that  prevails  among  the 
annelids  is  bilateral  twinning  or  isolation  of  the  right 
and  left  side  of  the  single  embryonic  axis.  This  type  of 
twinning  was  only  occasionally  met  with  in  the  echino- 
derms,  where  the  prevailing  mode  is  one  which  involves 
double  or  triple  gastrulation,  or  the  formation  of  paired 
embryonic  axes.  Since  both  of  these  modes  of  twinning 
are  common  throughout  the  animal  kingdom,  it  seems 
well  to  have  shown  them  in  unequivocal  form  in  groups 
where  the  processes  are  clear  and  the  causes  known  at 
least  to  a  high  degree  of  probability.  It  is  my  belief 
that  the  analysis  of  the  double-monster  situation  in  the 
earthworms  practically  settles  the  controversy  as  to 
the  mode  of  origin  of  vertebrate  conjoined  twins,  and 
this  chapter  is  therefore  placed  well  toward  the  beginning 
of  the  book  because  of  its  bearing  on  the  equivalent 
conditions  in  the  vertebrates  about  which  there  have  been 
widely  divergent  opinions. 


CHAPTER  IV 
ONE-EGG  TWINS  IN  FISHES 
INTRODUCTION 

The  existence  of  one-egg  twins  and  double  monsters 
in  fishes  has  been  known  for  a  long  time.  Possibly  the 
earliest  reference  in  the  literature  to  this  subject  is 
that  of  Aldrovandus  (1642)  in  his  Monstrorum  Historia. 
Since  then  probably  no  less  than  a  hundred  papers  and 
monographs,  describing  one  or  more  specimens  of  fish 
exhibiting  some  degree  of  duplicity,  have  been  published. 
Most  of  these  authors  have  apparently  been  under  the 
impression  that  they  were  reporting  some  new  and 
strange  monstrosity  and  their  accounts  have  been 
superficial.  A  number  of  them  have  secured  considerable 
collections  of  eggs  containing  twins  or  double  monsters, 
and  practically  the  same  series  of  monstrous  types  has 
been  shown  to  appear  in  all  representative  collections. 

As  to  the  frequency  with  which  twirining  occurs  in 
fishes  we  have  only  a  small  amount  of  evidence.  Rauber 
found  two  double  monsters  among  1,000  trout  eggs; 
and  one  in  325  pike  eggs;  Coste  found  over  100  double 
monsters  in  about  400,000  eggs  of  various  species; 
Lereboullet  found  222  double  monsters  in  203,962  eggs 
of  the  pike.  The  percentage  of  twins  is  so  small  that 
the  collection  of  an  adequate  series  of  specimens  must 
be  a  task  of  some  moment.  Among  the  kinds  of  fishes 
in  which  double  monsters  have  been  described  we  may 
mention  the  following:    sharks,  skates  and  rays,  lung- 

38 


ONE-EGG  TWINS  IN  FISHES  39 

fishes,  salmon,  trout,  mackerel,  perch,  pike,  and  killifishes. 
The  trout  is  by  all  odds  the  favorite  type  and  no  less 
than  twenty-five  separate  reports  of  duplicity  in  trout 
eggs  have  appeared.  Some  significance  attaches  to 
the  fact  that  the  trout  shows  a  higher  incidence  of 
duplicity  than  other  species.  In  the  first  place  the 
trout  is  the  favorite  game  fish  of  the  world  and  is  more 
commonly  reared  artificially  in  hatcheries  than  is  any 
other  fish.  This  alone  would  account  for  the  more 
frequent  observation  of  monstrosities  of  various  sorts, 
but  would  not  account  for  the  higher  percentage  of 
double  monsters  and  twins.  It  seems  probable  that  the 
trout,  being  a  fish  of  the  cold,  pure,  and  thoroughly 
oxygenated  waters  of  streams  and  spring- fed  lakes, 
is  more  abnormally  environed  during  development  in 
crowded  hatcheries  than  would  be  most  other  fishes. 
The  key  to  the  cause  of  twinning  doubtless  lies  in  this 
circumstance,  as  we  shall  later  attempt  to  show. 

Of  the  various  authors  who  have  contributed  to  our 
knowledge  of  twinning  in  fishes  the  following,  placed  in 
their  chronological  order,  seem  to  me  to  deserve  especial 
attention:  Lereboullet  (1855  and  1861),  Knoch  (1873), 
Rauber  (1877,  1878,  and  1879),  Klausner  (1890),  Windle 
(1895),  Kopsch  (1899).  Schmitt  (1901  and  1902), 
Gemmill  (1901  and  1912),  Stockard  (1921). 

MODES   OF   ORIGIN   OF   FISH   TWINS 

Various  classifications  of  fish  twins  have  been  given 
by  different  authors.  That  of  Gemmill  (191 2)  seems 
the  most  satisfactory  of  those  hitherto  published. 

Whatever  be  the  causation,  we  may  recognize  in  vertebrates 
generally,  four  somewhat  different  modes  of  origin,  whether  tor 


40  THE  PHYSIOLOGY  OF  TWINNING 

double  (and  multiple)  monstrosities,  or  for  double  (and  multiple) 
unioval  separate  embryos. 

These  different  modes  are:  (i)  The  appearance  of  two  (or 
more)  embryonic  rudiments  on  a  single  blastoderm.  (2)  The 
presence  in  the  egg  of  two  (or  more)  separate  blastoderms.  (3) 
Fission  or  dichotomy  on  the  part  of  a  single  embryonic  rudiment. 
(4)  Formation  of  certain  axial  structures  in  two  parallel  sets  on  a 
single  embryonic  rudiment. 

Modes  1  and  2  are  doubtless  exactly  equivalent  to 
the  similar  modes  described  in  chapter  i  for  the  starfish. 
Modes  3  and  4  are,  I  believe,  merely  different  degrees  of 
the  same  process  of  longitudinal  fission,  which  involves 
the  more  or  less  complete  bilateral  separation  of  the 
two  sides  of  the  bilateral  blastoderm. 

Gemmill  considers  that  the  first  mode  is  universal 
for  fishes,  that  the  second  is  represented,  in  fishes,  only 
by  a  single  instance  in  the  literature  (Klausner's  case 
cited  below),  and  that  the  third  and  fourth  modes  of 
origin  are  extremely  rare  in  fishes. 

There  is  little  doubt,  I  believe,  that  separate  one-egg 
twins  may  and  do  originate  by  means  of  both  of  the 
first  two  modes.  Several  writers  have  described  instances 
of  germ-ring  stages  in  which  there  were  two  or  more 
embryonic  shields.  Rauber,  especially,  has  given  us 
unequivocal  examples  of  this  mode  of  origin  as  shown 
in  Figures  23  and  24.  Such  examples  as  these  are 
evidently  instances  quite  completely  equivalent  to  the 
commonest  type  of  twinning  in  the  starfish,  involving  a 
loss  of  axiation  of  the  single  blastoderm  and  the  produc- 
tion of  two  or  more  regions  of  gastrulation  instead  of 
the  original  one.  Rauber's  second  figure  (Fig.  24)  shows 
on  the  right  side  what  I  believe  is  an  early  stage  of  a 
double  monster  resulting  from  the  dichotomy  of  an  origi- 


ONE-EGG  TWINS  IN  FISHES 


4i 


nally  single  embryonic  shield.  It  is  not,  as  Gemmill 
believes,  the  product  of  the  fusion  of  two  adjacent  em- 
bryonic shields. 

Twins  originating  from  plural  invaginations  of  the 
margin  of  the  germ  ring  must  inevitably  come  to  lie 
parallel  to  each  other  with  heads  pointing  in  the  same 


Figs.  23,  24. — Germ-ring  stages  of  twin  trout  embryos.  Fig.  23,  an 
embryo  with  two  equivalent  embryonic  shields,  destined  to  form  separate 
twins.  Fig.  24,  an  embryo  with  two  embryonic  shields,  one  destined  to 
form  a  single  separate  individual  and  the  other  a  two-headed  double 
monster.     (After  Rauber.) 


direction.  They  must  also  have  a  common  yolk  sac 
and  yolk  stalk.  Such  twins  could  therefore  never 
become  fully  separate. 

The  question  arises  as  to  whether  it  would  be  possible 
for  fish  twins  to  arise  in  such  a  way  that  they  would 
be  separate  when  hatched.  Klaussner  (1S90)  cites  one 
case  of  fish  twins  which  might  possibly  have  become 
separate  after  hatching.  This  pair  of  twins  was  found 
lying  side  by  side  on  one  egg  but  with  heads  pointed  in 
opposite  directions.  Obviously  such  a  condition  could 
not  have  arisen  as  product  of  the  invagination  of  two 


42  THE  PHYSIOLOGY  OF  TWINNING 

points  upon  one  germ  ring.  They  therefore  must  have 
come  from  two  separate  blastoderms.  The  two  blasto- 
derms probably  arose  through  the  physiological  isolation 
of  the  first  two  blastomeres,  as  so  frequently  happens  in 
parthenogenetic  Patiria  eggs.  This  mode  of  origin  of 
fish  twins  must  be  extremely  rare,  for  no  other  such 
cases  are  on  record. 

Gemmill  is  of  the  opinion  that  the  third  mode  of 
twinning  is  found  in  fishes  only  in  connection  with  "the 
peculiar  and  imperfect  doubling  characteristic  of  the 
hemididymous  condition."  Two  phases  of  hemididymus 
are  distinguished:  (a)  mesodidymus,  in  which  there  is 
apparent  doubling  of  the  middle  region  while  the  anterior 
and  posterior  ends  remain  single;  (b)  katadidymus,  in 
which  the  anterior  ends  remain  single  and  the  posterior 
ends  are  double. 

As  compared  with  anadidymus,  in  which  the  anterior 
end  is  double  while  the  posterior  end  is  single,  the  two 
forms  of  hemididymus  are  extremely  uncommon,  and  are 
rarely  if  ever  found  in  advanced  embryos.  They  seem 
to  be  due  to  a  mechanical  pulling  apart  of  the  bilateral 
primordia  during  germ-ring  overgrowth.  Kopsch  was 
able  to  get  katadidymus  in  trout  eggs  by  injuring  the 
blastoderm  at  the  posterior  end  of  the  embryo.  Knoch 
found  instances  of  katadidymus  in  cases  where  eggs  were 
rather  roughly  handled  by  violent  stirring.  In  general 
the  condition  is  more  like  spina  bifida  than  true  twinning 
and  may,  I  believe,  be  dismissed  without  further  con- 
sideration. Gemmill's  fourth  mode  of  twinning  is 
illustrated  for  the  fishes  by  a  single  example  of  a  salmon 
embryo,  reported  by  Barbieri  (1906),  in  which  there  is  a 
marked  tendency  for  ventral  organs  to  show  greater 


ONE-EGG  TWINS  IN  FISHES  43 

duplicity  than  dorsal  ones.  Gemmill  interprets  this 
condition  as  the  result  of  the  origin  of  two  axes  in 
a  single  embryonic  shield  accompanied  by  secondary 
fusion  of  dorsal  structures. 

It  should  be  emphasized  that  the  last  three  modes 
of  twinning,  as  interpreted  by  Gemmill,  are  extremely 
rare  among  fishes  and  that  none  of  them  have  been  noted 
by  those  observers  who  have  made  large  and  represent- 
ative collections.  Cases  of  hemididymus  have  been 
observed  mainly  in  quite  early  germ-ring  stages  and  there 
is  ground  for  believing  that  separations  observed  and 
figured  are  merely  cases  of  mechanical  dehiscence  more 
or  less  temporary  in  character.  The  case  of  Barbieri, 
cited  above,  is  so  extraordinary  and  so  unlikely  that,  until 
confirmatory  instances  have  been  described,  it  seems 
hardly  worth  while  to  speculate  about  its  significance. 

If  we  put  aside  the  rare,  exceptional,  and  poorly 
understood  types  of  duplicity  in  fishes  we  find  that  there 
remain  only  two  definite  types  of  twinning: 

a)  Separate  twins,  in  which  the  entire  bodies  of  the 
two  individuals  are  separate  with  the  exception  of 
unions  in  the  yolk-sac  region.  On  account  of  mechanical 
limitations  it  does  not  appear  possible  for  fish  twins 
arising  from  a  single  blastoderm  to  be  entirely  separate. 
Morphologically  speaking,  however,  such  individuals  are 
equivalent  to  separate  one-egg  twins  in  mammals  and 
will  be  considered  here  as  separate  twins. 

b)  Conjoined  twins  of  the  anadidymus  type,  in  which 
there  is  anterior  duplicity  for  a  greater  or  less  distance 
and  posterior  singleness.  This  is  the  only  type  of  fish 
double  monster  described  by  those  who  have  made  large 
collections. 


44  THE  PHYSIOLOGY  OF  TWINNING 

These  two  standard  classes  of  fish  twins  will  now  be 
considered  in  detail. 

SEPARATE   TWINS 
MODE   OF   ORIGIN 

All  of  the  writers  who  have  attempted  to  formulate 
theories  of  the  mode  of  origin  of  fish  twins  in  general 
have  offered  the  same  explanations  for  both  separate  and 
conjoined  twins.  GemmilPs  view  is  representative  of 
all  such  views  and  is  expressed  as  follows: 

The  recorded  observations  indicate  that  double-monster 
fishes  (including  those  united  by  yolk  sac  only)  always  arise  on  a 
single  yolk  and  from  a  single  blastoderm  at  the  margin  of  which 
two  more  or  less  separate  centers  of  gastrulation  and  embryo- 
formation  have  appeared. 

The  twin  centers  of  embryo-formation  mentioned  above  may 
be  classed  in  two  groups  (a)  and  (b),  according  to  the  distance 
which  separates  them  from  one  another,  (a)  In  the  first  and  most 
important  group  the  interval  is  not  too  great  to  prevent  approxima- 
tion and  union  of  the  two  embryonic  axes  from  taking  place 

during  the  natural  course  of  their  growth  in  length (b)  In 

the  second  group,  the  twin  centers  of  embryo-formation,  are  so 
far  apart  that  there  is  no  compelling  influence  of  the  kind  described 
above  which  would  lead  to  the  approximation  and  union  of  their 
growing  embryonic  axes.  Accordingly  the  twin  bodies  remain 
separate,  except  for  the  adventitious  union  supplied  by  the 
layers  forming  the  common  yolk  sac. 

I  see  no  reason  to  doubt  that  at  least  many  of  the 
truly  separate  fish  twins  arise  in  the  manner  described 
above.  But  I  have  reason  to  believe  that  at  least  some 
separate  twins  and  all  truly  conjoined  twins  arise  by 
partial  dichotomy  or  by  complete  fission  of  the  right-  and 
left-hand  primordia  of  a  single  axis.  In  other  words, 
there  may  be  two  types  of  separate  twins,  those  that 


ONE-EGG  TWINS  IN  FISHES  45 

originate  from  separate  embryonic  axes  or  centers  of 
gastrulation  and  those  that  originate  from  the  fission 
of  a  single  embryonic  axis.  As  will  be  shown  below,  all 
true  anadidymi  must  be  viewed  as  incomplete  fission 
products  of  a  single  axis.  If  this  view  be  true,  some 
separate  twins  may  well  belong  to  the  same  series  as 
the  anadidymi,  but  not  in  the  sense  of  Gemmill,  who 
believes  that  all  arise  as  separate  centers  of  gastrulation 
and  that  conjoined  twins  are  products  of  fusion. 

ORIGIN   OF  AUTOSITE  AND  PARASITE   TWINS 

It  is  just  here  that  we  may  profitably  turn  back  to 
the  conditions  described  for  the  starfish.  It  will  be 
remembered  that  very  frequently  the  original  axis  of  the 
blastula  is  only  partially  obliterated  so  that  the  main 
basal  area,  or  region  of  gastrulation,  persists  while  one  or 
more  minor  basal  areas  may  have  been  established.  Such 
minor  areas  give  rise  to  secondary  archentera,  smaller  in 
size  and  situated  at  regions  where  archentera  would  not 
be  expected,  sometimes  appearing  at  the  opposite  pole  or 
apical  end  of  the  blastula.  In  the  starfish  these  second- 
ary archentera  persist  for  a  time  but,  in  most  cases,  are 
completely  inhibited  and  subsequently  become  pinched 
off  and  absorbed.  In  a  few  cases  the  secondary  archen- 
teron  may  be  nearly  as  large  and  active  as  the  primary, 
and  may  persist  as  long  as  the  larva  lives.  Vow  in  the 
fishes  something  very  much  like  this  probably  takes 
place.  When  the  inhibition  has  been  insufficient  cut  inly 
to  obliterate  the  original  axiate  relations  of  the  blasto 
derm,  the  original  gastrulation  area  persists;  hut  a 
secondary  area  arises  probably  at  the  opposite  side  of 
the  blastoderm.     This  secondary  area,  after  beginning 


46  THE  PHYSIOLOGY  OF  TWINNING 

to  invaginate  and  to  form  an  axis,  may  become  inhibited 
through  the  much  more  active  growth  of  the  primary 
axis  and  may  become  obliterated.  Stockard,  for  example, 
noted  a  number  of  cases  in  which  secondary  embryonic 
shields  appeared  but  no  accessory  embryos  were  formed. 
If,  however,  the  secondary  embryonic  shield  is  allowed 
to  develop  far  enough  it  cannot  be  completely  suppressed 
by  the  primary  embryo  and  a  secondary  or  subordinate 
embryo  will  develop.  Such  an  embryo  will  always,  I 
believe,  exhibit  some  evidences  of  being  inhibited,  such 
as  small  size,  cyclopia,  and  other  defects.  The  primary 
embryo,  as  in  the  starfish  larvae,  may  grow  quite 
normally,  apparently  suffering  no  detriment  from  the 
presence  of  the  secondary  embryo,  until  they  have 
mutually  surrounded  the  yolk  sac  and  thus  come  to  lie 
belly  to  belly,  attached  by  means  of  the  vitelline  tissues. 
When  this  happens  the  larger  primary  embryo  tends  to 
grow  around  and  absorb  the  smaller  secondary  embryo 
and  frequently  succeeds  almost  completely  in  doing  so. 
The  larger  larva  is  here  the  autosite  and  the  smaller 
one  the  parasite  (Figs.  25  and  26).  It  is  my  opinion 
that  the  condition  of  autositism  and  parasitism  in  fishes 
practically  always  arises  in  this  fashion.  Cases  of  false 
autosites  and  parasites  have  been  described  by  Stockard 
in  connection  with  true  conjoined  twins  derived  by  the 
separation  of  parts  of  a  single  embryonic  axis.  This 
condition  does  not  concern  us  here,  for  we  are  dealing 
with  twins  derived  by  plural  gastrulation. 

ORIGIN   OF   TRUE   DUPLICATE    TWINS 

Harking  back  once  more  to  the  starfish  situation, 
it  may  be  recalled  that  a  very  common  type  of  twin 


ONE-EGG  TWINS  IN  FISHES 


47 


larva  was  one  in  which  the  original  polarity  of  the  blastula 
had  been  largely  obliterated  and  in  which  two  new  twin 
gastrulation  areas  had  arisen,  each  of  which  is  a  secondary 
area  and  each  quite  definitely  a  mirror-image  duplicate 
of  the  other.  Neither  one  is  primary  to  the  other  and 
neither  one  tends  to  inhibit  the  development  of  the  other. 


Hi^. 


Figs.  25,  26. — Typical  examples  of  trout  twins  of  the  "autosite- 
parasite"  variety.  In  both  cases  the  autosite  is  nearly  normal  and  the 
parasite  decidedly  subnormal.     (After  Stockard.) 

They  grow  at  equal  rates  and  form  identical  twin  axes. 
The  condition  may  be  said  to  be  due  to  the  physiological 
isolation  of  the  two  halves  of  the  original  blastoderm. 

Now  just  such  a  process  as  this  takes  place,  I  believe, 
in  the  fish  blastoderm  and  accounts  for  the  numerous 
cases  of  duplicate  twins  attached  only  by  the  yolk  sac 
or  by  parts  of  the  lateral  body  wall.  The  two  individuals 
are  each  complete  and  normal  and  of  approximately 
equal  size.  Examples  of  such  duplicate  twins  arc  seen 
in  Figures  27  and  28.     The  differences  with  regard  to  the 


48 


THE  PHYSIOLOGY  OF  TWINNING 


relative  positions  of  these  twins  and  their  points  of 
attachment  depend,  I  believe,  upon  how  far  apart  on  the 
blastoderm  the  twin  gastrulation  areas  occur.  It  may 
be  recalled  that,  in  the  duplicate  twin  starfish  larvae, 
the  archentera  sometimes  occurred  closely  side  by  side, 
and  sometimes  they  occurred  as  far  apart  as  possible. 


Figs.  27,  28. — Typical  "separate"  trout  twins  arising  from  double 
symmetrical  gastrulation.  Fig.  27,  a  pair  of  identical  twins,  both  normal. 
Fig.  28,  a  pair  of  such  twins,  one  of  which  is  somewhat  smaller  and 
defective  in  the  head  region.     (After  Stockard.) 


Doubtless  the  same  is  true  for  the  fishes.  If  the  two 
individuals  arise  close  together  we  would  expect  them 
to  be  united  by  their  inner  sides;  but  if  they  arise  on 
opposite  sides  of  the  blastoderm,  they  should  be  facing 
one  another  belly  to  belly.  And  there  would  be  many 
intermediate  stages.  Such  unions  never  involve  any 
more  than  mere  external  fusions,  axial  elements  such 
as  notochord,  neural  tube,  and  alimentary  tract  never 
being  united. 


ONE-EGG  TWINS  IN  FISHES  49 

When  two  individuals  have  arisen  from  two  closely 
approximated  embryonic  axes  they  may  be  crowded 
together  laterally  so  closely  that  the  structures  of  the 
inner  sides,  such  as  the  pectoral  and  pelvic  fins,  may  come 
to  be  more  or  less  fused  and  crowded  out  of  place;  but 
this  situation  is  only  to  be  expected  as  the  result  of  such 
close  quarters.  This  kind  of  fusion,  however,  is  quite 
different  in  principle  from  the  sort  of  primary  fusion 
that  Gemmill  and  others  invoke  in  order  to  account  for 
double  monsters. 

CAUSE  OF  SUBNORMAL  DEVELOPMENT  OF  ONE  TWIN 

When  two  embryos  arise  by  plural  gastrulation  one 
of  the  individuals  is  often  subnormal  throughout  its 
life.  Is  its  subnormal  condition  due  to  an  inhibition 
exerted  by  the  larger  embryo,  or  is  it  the  result  of  being 
subordinate  from  the  time  of  its  origin?  My  theory  is 
that  in  many  cases  the  original  axiate  relations  have 
persisted  to  some  extent;  that  on  this  account  any 
individual  arising  from  a  secondary  gastrulation  area 
was  from  the  first  subordinate  and  inhibited  owing  to 
its  origin  from  a  region  that  still  belonged,  at  least 
partially,  to  the  original  axis  of  the  single  individual. 
In  extreme  cases  such  a  secondary  axis  is  rather  promptly 
suppressed  when  the  primary  individual  regains  its 
normal  growth  momentum.  In  less  extreme  cases  the 
secondary  individual  is  able  to  maintain  some  degree 
of  independence,  but  is  handicapped  at  first  by  its 
slower  start  and  secondarily  by  its  contact  with  the  Larger, 
more  vigorous  individual. 

In  so  far  as  the  mode  of  origin  of  separate  fish  twins 
is  concerned,  with  the  possible  exception  of  those  with 


50  THE  PHYSIOLOGY  OF  TWINNING 

situs  inversus  viscerum,  there  is  no  disagreement  between 
Gemmill  and  his  followers  and  myself,  but  I  cannot 
accept  their  theory  of  the  origin  of  conjoined  twins, 
which  are  really  much  commoner  in  fishes  than  are 
separate  twins  and  have  excited  much  more  comment 
and  interest  than  the  latter.  The  next  chapter  is  to  be 
devoted  exclusively  to  conjoined  twins  or  true  double 
monsters,  their  causes,  and  mode  of  origin. 


CHAPTER  V 

DOUBLE  MONSTERS  OR  CONJOINED  TWINS 

IN  FISHES 

CLASSIFICATION   AND   ANATOMICAL   STRUCTURE 
OF   CONJOINED   TWINS 

The  earliest  adequate  classification  of  conjoined 
fish  twins  is  that  of  Windle  (1895),  who  concerns  himself 
only  with  the  external  evidences  of  duplicity.  He 
recognizes  eleven  classes  of  trout  twins  characterized 
by  the  following  structural  peculiarities : 

1.  Three  eyes  of  the  same  size  (the  least  manifestation 
of  duplicity  noted  by  any  of  the  authors). 

2.  Three  eyes,  the  median  being  larger  than  either 
of  the  lateral  ones. 

3.  Four  equal-sized  eyes. 

4.  Two  heads,  the  duplicity  extending  as  far  back 
as  the  otic  region. 

5.  Duplicity  extends  to  the  region  of  the  pectoral  fins. 

6.  Duplicity  extends  to  the  posterior  border  of  the 
yolk  sac,  the  caudal  extremity  of  the  fishes  being  quite 
single. 

7.  Duplicity  extends  a  short  distance  behind  the 
posterior  border  of  the  yolk  sac,  but  the  caudal  extremity 
is  quite  single. 

8.  Duplicity  extends  to  the  posterior  border  of  the 
yolk  sac.  Behind  this  there  are  two  caudal  extremities 
overlapping  one  the  other  and  firmly  united  by  their 
contiguous  aspects. 

Si 


52  THE  PHYSIOLOGY  OF  TWINNING 

9.  Union  by  caudal  extremities  alone  (not  seen  by 
Windle  himself,  but  noted  by  early  writers). 

10.  Union  by  ventral  aspects  at  the  site  of  the 
attachment  of  the  yolk  sac. 

11.  Parasites.  (All  cases  in  which  one  individual 
is  distinctly  smaller  than  the  other  and  strongly  united 
to  it.) 

Windle's  classification,  though  dealing  only  with 
superficial  characteristics,  emphasized,  rightly  I  believe, 
the  degree  of  duplicity  rather  than  that  of  union  in 
all  cases  of  incompletely  double  individuals.  His  first 
seven  classes  are,  in  my  opinion,  true  instances  of 
anadidymi  or  products  of  incomplete  fission  of  a  single 
bilateral  primordium.  The  remaining  four  classes  repre- 
sent twins  derived  from  separate  embryonic  axes,  but 
more  or  less  secondarily  fused  by  external  parts.  These, 
according  to  our  theory,  are  separate  twins  and  do  not 
logically  fall  into  the  category  of  double  monsters. 
Windle  has  no  theory  of  the  origin  of  these  forms  to  advo- 
cate, his  paper  being  purely  descriptive. 

CLASSIFICATION   OF  GEMMILL   (1891) 

In  his  early  paper,  a  summary  read  before  the  Royal 
Society  of  London  over  thirty  years  ago,  Gemmill  gave 
us  a  very  painstaking  classification  of  a  large  collection 
of  trout  twins  and  double  monsters,  based  on  the  study 
of  the  internal  relations  of  the  connected  individuals. 
It  will  be  noted  that  he  emphasizes  union  rather  than 
duplicity.     His  classification  is  as  follows: 

Type  1 .  Union  in  head  region : 

a)  The  twin  brains  united  at  the  mesencephalon. 

b)  The  twin  brains  united  at  the  medulla  oblongata. 


DOUBLE  MONSTERS  IN  FISHES  53 

Type  2.  Union  in  pectoral  region: 

a)  The  pectoral  fins  absent  011  adjacent  sides. 

b)  The  pectoral  fins  present,  but  united  on  adjacent  sides. 
Type  3.   Union  behind  the  pectoral  region: 

a)  The  twin  bodies  united  at  a  considerable  distance  in 
front  of  the  vent. 

b)  The  twin  bodies  united  close  to  the  vent. 

Type  4.  Union  by  yolk  sac  only. 

The  first  three  types  should  be  classed  as  conjoined 
twins  or  double  monsters  and  the  fourth  type  as  sepa- 
rate or  duplicate  twins.  The  series  is  evidently  a  very 
complete  one  in  which  every  degree  of  duplicity  of  the 
primary  axis,  from  a  slight  twinning  in  the  most  anterior 
region  to  complete  separation,  is  found.  There  are  no 
cases  of  double  tails  unless  the  twins  are  entirely  separate. 

By  far  the  commonest  type  of  double  monster  is 
that  in  which  the  process  of  dichotomy  extends  well 
into  the  abdominal  region,  but  not  past  the  pelvic  region. 
Such  an  individual  is  shown  in  Figure  29  (p.  54).  As 
a  rule  individuals  showing  this  degree  of  duplicity  show 
approximately  equal  development  in  the  two  anterior 
components.  When  the  duplicity  extends  farther  toward 
the  posterior  end,  as  in  Figure  30,  there  is  a  greater 
tendency  for  one  component  to  be  abnormal.  Cases  in 
which  the  duplicity  is  confined  to  the  most  anterior  struc- 
tures are  relatively  infrequent.  Such  a  type  is  shown 
in  Figure  31,  in  which  the  duplicity  is  confined  to  the 
forebrain,  and  there  is  but  one  median  eye,  belonging 
partly  to  one  and  partly  to  the  other  incompletely 
double  head.  There  are  probably  cases  of  twinning  in- 
volving even  less  than  this,  but  no  good  figures  of  such 
were  to  be  found. 


54 


THE  PHYSIOLOGY  OF  TWINNING 


Although  it  seems  to  me  that  GemmilPs  data  almost 
automatically  speak  forth  the  fission  theory  of  twinning, 
it  is  perfectly  obvious  that  throughout  he  has  in  mind 
exactly  the  opposite  process.  He  always  speaks  of  the 
uniting  of  paired  parts  or  regions  into  single  elements. 
All  of  his  types  of  twins  are  interpreted  as  cases  of  more 


Figs.  29-31. — Typical  examples  of  trout  double  monsters,  arising 
from  the  partial  fission  or  dichotomy  of  a  single  embryonic  axis.  Fig.  29, 
the  commonest  type  of  trout  twin,  exhibiting  situs  inversus  viscerum. 
Fig.  30,  a  case  in  which  the  fission  products  are  unequal,  one  being 
decidedly  subnormal.  Fig.  31,  a  rather  rare  type  of  minimal  twinning, 
the  dichotomy  being  confined  to  the  most  anterior  structures,  the  eyes 
and  the  forebrain.  A  horizontal  section  through  such  a  type  is  shown  in 
Fig.  57.     (Figs.  29  and  30  after  Stockard,  Fig.  31  after  Gemmill.) 

or  less  complete  fusion  of  two  individuals.  He  implies 
that  conjoined  twins  are  derived  from  originally  separate 
individuals  that  have  been  brought  together  by  the 
mechanism  of  germ-ring  closure  and  have  fused  more 
or  less  completely.  This  idea  naturally  leads  to  a  far- 
reaching  conception  of  the  mode  of  formation  of  bilater- 


DOUBLE  MONSTERS  IN  FISHES  55 

ally  symmetrical  organisms:  the  now  largely  discredited 
concrescence  theory. 

In  his  more  general  work  on  the  teratology  of  fishes 

Gemmill  (1872)  adheres  to  his  original  idea  of  double 
monsters  as  products  of  partial  fusion  of  originally 
separate  embryos  and  accounts  for  the  degrees  of 
separateness  of  the  anterior  parts  in  an  ingenious  way. 
He  considers  the  germ  ring  "as  a  stock,  able  to  give  rise 
vegetatively,  so  to  speak,  to  more  than  one  embryo." 
As  a  rule  only  one  embryonic  shield  arises  on  the  germ 
ring,  but  the  germ  ring  is  believed  to  be  potentially 
capable  of  giving  rise  to  accessory  embryonic  shields 
at  any  distance  from  the  first.  When  in  this  way  two 
shields  arise,  the  level  of  the  point  of  union  "varies 
directly  with  the  original  distance  between  the  two 
centers  of  embryo  formation." 

In  view  of  the  fact  that  the  validity  or  non-validity 
of  this  "budding  theory"  of  twinning  in  fishes  depends 
on  a  proper  interpretation  of  the  nature  of  the  germ 
ring  and  the  mode  of  formation  of  the  embryonic  axis, 
the  reader  will  doubtless  be  indulgent  enough  to  allow 
us  to  present  a  summary  of  the  evidence  on  this  point. 

THE    CONCRESCENCE    THEORY   AND    THE    INTERPRETATION 
OF   CONJOINED   TWINS   IN   FISHES 

According  to  the  concrescence  theory  the  embryonic 
shield  represents  merely  the  head  end  of  the  future 
embryo,  while  the  lateral  halves  of  the  rest  of  the  body 
are  separated  from  each  other  and  are  represented  in 
the  germ  ring.  As  the  germ  ring  passes  the  equator  of 
the  yolk  it  concresces  to  form  the  bilateral  elements 
of  the  embryo.     This  is  evidently  the  view  originally 


56  THE  PHYSIOLOGY  OF  TWINNING 

taken  by  Gemmill,  and  if  true  would  apparently  account 
for  the  partial  uniting  of  two  embryonic  axes  into  one 
in  the  case  of  conjoined  twins.  Since  the  germ  ring  is 
supposed  to  represent  the  two  sides  of  the  axis,  what 
would  happen  if  a  second  head  or  embryonic  shield  arose 
to  the  right  or  to  the  left  of  the  original  shield  ?  Obvi- 
ously the  two  heads  would  be  in  competition  for  that 
region  of  the  germ  ring  between  them.  The  germ  ring 
is  gradually  taken  up  partly  by  one  embryo  and  partly 
by  the  other.  They  would  therefore  have  separate 
inner  sides  as  long  as  the  common  region  of  the  germ 
ring  lasted,  but  they  would  sooner  or  later  use  this  up 
and  neither  embryo  would  have  any  more  material  for 
its  inner  half.  Beyond  this  point,  therefore,  the  two 
outer  halves  of  the  germ  ring,  one  half  belonging  to  one 
embryo  and  the  other  to  the  other,  would  concresce  in 
the  median  line  to  form  the  single  or  untwinned  part  of 
the  body.  Thus  we  would  readily  obtain  embryos  with 
two  anterior  regions  and  a  single  or  common  posterior 
region.  The  farther  apart  the  embryonic  shields  arise 
on  the  germ  ring  the  greater  the  extent  of  the  twinned 
or  double  region.  If  embryonic  shields  arise  on  opposite 
sides  of  the  germ  ring  the  twins  would  be  entirely 
separate,  except  that  they  have  a  common  yolk  stalk, 
a  condition  that  could  hardly  be  avoided  since  the  two 
embryos  must  have  a  common  blastopore  and  are  on  a 
single  yolk  sac.  This  all  sounds  reasonable  enough  on 
the  basis  of  the  concrescence  theory,  but  unfortunately 
this  theory  is  now  practically  discredited.  No  longer 
can  it  be  maintained  that  the  germ  ring  represents  the 
separated  right  and  left  halves  of  the  embryonic  axis, 
for  it  has  been  proved  experimentally  by  several  reliable 


DOUBLE  MONSTERS  IN  FISHES  57 

workers  that  the  germ  ring  does  not  concresce  to  form 
the  embryonic  axis. 

KOPSCH'S   VIEW   OF   EMBRYO   FORMATION' 

In  a  very  able  monograph  entitled  Untersuchungen 

iiber  Gastrulation  und  Embryo-bildungen  bei  den  Chordalcn, 
Kopsch  (1904)  has  brought  out  the  following  facts: 

a)  If  with  a  hot  needle  a  portion  of  the  germ  ring 
near  an  early  embryonic  shield  is  killed,  such  injury  has 
no  effect  upon  the  organization  of  the  future  embryo 
but  merely  results  in  the  formation  of  a  scar  in  the  tissue 
outside  of  the  embryo  proper.  This  was  true  no  matter 
how  close  to  the  embryonic  shield  the  injury  was  made. 

b)  If  the  posterior  end  of  the  embryonic  shield  is 
injured,  the  entire  embryonic  body  is  formed  except 
the  tail  and  adjacent  tissues.  This  means  that  the 
whole  body  is  represented  in  the  original  embryonic 
shield  and  is  not  built  up  by  germ-ring  concrescence. 

c)  The  embryonic  shield,  as  soon  as  it  is  definitely 
established,  contains  the  complete  embryonic  axis  from 
head  to  tail.  At  first  the  tail  proper  is  represented  only 
by  a  tail-bud,  which  begins  to  grow  out  even  before  the 
germ  ring  has  completed  its  envelopment  of  the  yolk. 

d)  The  only  evidence  of  any  concrescence  at  all  is 
seen  in  connection  with  the  formation  of  the  Knopf  or 
tail-bud  region.  At  first  this  region  appears  to  aris 
at  the  two  outer  margins  of  the  thickened  portion 
destined  to  form  the  embryonic  shield.  These  bilateral 
regions  appear  to  grow  in,  apparently  more  by  migration 
of  cells  than  by  gross  concrescence,  to  form  a  median 
posterior  area  of  the  shield  which  is  essentially  the 
tail-bud. 


58  THE  PHYSIOLOGY  OF  TWINNING 

e)  As  soon  as  this  Knopf  region  is  organized  the  axis 
of  the  embryo  is  complete  and  all  increase  in  the  length 
of  the  axis  occurs  in  a  growing  zone  between  the  head  and 
the  tail-bud. 

That  this  is  not  merely  one  man's  interpretation  of 
these  conditions  is  seen  in  the  fact  that  essentially  the 


Figs.  32,  2>2>- — Diagrams  showing  Kopsch's  interpretation  of  the 
mode  of  embryo  formation  in  the  teleost  fishes.  K,  the  Kopf;  R,  the 
Knopf.     For  explanation  see  text.     (From  Kopsch.) 

same  position  is  taken  by  Morgan,  Virchow,  Sumner, 
Jablonowski,  and  other  workers  on  teleost  embryology. 
Professor  F.  R.  Lillie,  who  has  given  much  attention  to 
this  matter,  considers  the  position  of  Kopsch  entirely 
sound.  I  propose  then  to  accept  this  consensus  of 
judgment  about  the  nature  of  concrescence  and  the 
formation  of  the  embryo  in  teleosts.  Kopsch's  conclu- 
sions on  these  problems  are  as  follows. 

When  the  embryonic  primordium  is  still  merely  a 
somewhat  thicker  sector  than  the  germ  ring  (Fig.  32), 
there  is  a  middle  region  (K)  the  Kopf  (or  head)  and  two 
laterally  arranged  areas  (R),  the  Knopf  (or  tail-bud). 
When  the  embryonic  shield  proper  is  formed  there 
appears  to  be  a  sort  of  migration  forward  and  toward 
the  median  line  of  the  cells  of  the  Kopf  region,  which  is 


DOUBLE  MONSTERS  IN  FISHES  59 

possibly  to  be  interpreted  as  concrescence.  Similarly 
the  Knopf  regions  come  together  to  form  a  single  median 
tail-bud,  which  completes  the  embryonic  axis  and  is  to 
be  considered  as  the  final  step  in  this  very  limited  proo 
of  concrescence  (Fig.  33).  Once  the  germinal  shield 
is  formed,  the  axis  is  to  be  considered  as  completed,  and 
no  farther  concrescence  is  possible  except  a  slight  migra- 
tion inward  of  some  of  the  adjoining  regions  of  the  germ 
ring  to  form  lateral  regions  of  the  embryo.  There  is  no 
opportunity  for  a  median  coalescing  of  outlying  regions 
of  the  germ  ring  to  form  any  part  of  the  embryonic  axis. 

If  this  view  is  valid,  it  has  a  most  important  bearing 
on  our  interpretation  of  conjoined  twins  in  the  fishes  as 
well  as  in  other  vertebrates.  Such  double-headed  and 
single-bodied  individuals  could  not  possibly  arise  from 
a  fusion  of  two  separate  embryonic  shields,  as  GemmiU's 
budding  theory  implies.  In  order  to  make  such  a  fusion 
possible  it  would  be  necessary  to  suppose  that  in  some 
extraordinary  way  the  inner  halves  of  each  twin  becomes 
obliterated  during  the  fusion  process  so  that  only  the  outer 
halves  remain,  and  that  these  outer  halves  come  together 
in  such  an  extremely  precise  way  as  to  fuse  notochord 
with  notochord,  neural  tube  with  neural  tube,  aorta  with 
aorta,  vein  with  vein,  intestine  with  intestine.  We  also 
would  have  to  suppose  that  at  the  time  of  fusion  the  two 
components  were  in  exactly  the  same  stage  of  develop- 
ment, for  otherwise  the  parts  that  come  together  in  the 
median  line  would  not  represent  equivalent  regions  in 
the  primary  axis,  and  there  could  be  no  exact  equivalence 
of  contribution  to  the  united  posterior  part  of  the  body. 

In  his  later  work  Gemmill  (191 2)  accepts  the  inter 
pretation  of  Kopsch  as  to  the  formation  of  the  embryonic 


60  THE  PHYSIOLOGY  OF  TWINNING 

axis  and  the  nature  of  the  germ  ring,  but  persists  in  the 
idea  that  double  monsters  are  the  result  of  a  fusion  of 
originally  separate  embryonic  axes.  His  own  statement 
makes  this  clear: 

Approximation  and  union  (of  the  twin  centers  of  embryo- 
formation)  are  due  to  the  factors  to  which  attention  was  directed 
above,  namely,  the  utilization  during  growth  of  the  blastodermic 
margins  near  the  primitive  streak,  and  the  slowness  of  expansion 
on  the  part  of  the  blastoderm  over  the  yolk  in  this  same  region 
The  twin  adjacent  axes  are  inevitably  brought  together  posteriorly 
through  the  disappearance  of  the  interval  between  them.  The 
process  may  be  called  one  of  primary  fusion,  in  contrast  with  a 
process  which  often  supervenes  later,  and  which  consists  in  the 
secondary  fusion  of  organs  or  structures  already  laid  down. 

Primary  fusion  takes  place  earlier  or  later,  i.e.,  in  the  head-, 
body-,  or  tail-region,  according  to  the  interval  which  separated 
the  embryonic  rudiments  when  they  first  appeared.  In  other 
words,  the  degree  of  duplicity  varies  directly  with  the  original 
distance  between  the  two  centres  of  embryo-formation.  When  the 
union  is  a  purely  lateral  or  an  approximately  lateral  one,  the  pos- 
terior united  part  finally  becomes  simply  and  perfectly  bilateral. 
This  takes  place  through  the  gradual  fusion  and  disappearance  of 
structures  belonging  to  the  left  and  right  halves  respectively  of  the 
right  and  left  component  embryos.  Thereafter  the  right  and  left 
halves  of  the  right  and  left  embryos  unite  naturally  to  form  a 
normal  bilateral  body  or  tail.  On  the  other  hand,  when  the 
twin  bodies  come  together  by  ventral  rather  than  by  bilateral 
union,  the  formation  posteriorly  of  a  perfect  single  body  or 
tail  becomes  impossible,  since  the  necessary  readjustments  of 
right  and  left  structures  in  the  twin  embryos  can  no  longer  take 
place. 

We  see  then  that  Gemmill  is  quite  uncompromising 
in  his  position  that  double  monsters  of  all  sorts  are  fusion 
products.  We  have  already  pointed  out  that  the  fusion 
theory  is  quite  untenable  and  shall  attempt  to  show  later 


DOUBLE  MONSTERS  IN  FISHES  6l 

that  the  dichotomy  or  fission  theory  much  more  nearly 
satisfies  the  conditions  observed. 

Undoubtedly  a  certain  amount  of  secondary  fusion 
of  external  adjacent  structures  does  occur,  as  for  example 
in  such  cases  as  that  described  by  Windle  in  which  the 
tails,  crossed  the  one  over  the  other,  are  united  for  a 
short  distance  by  the  fusion  of  the  ventral  side  of  one 
with  the  dorsal  side  of  the  other.  Apart  from  such 
obvious  cases  and  such  cases  as  those  referred  to  on  page 
48,  where  adjacent  identical  twins  come  to  fuse  side  to 
side  by  the  body  wall,  we  may  safely  abandon  the  fusion 
theory  of  the  origin  of  true  double  monsters. 

stockard's  theory  of  the  mode  of  origin  of 
twins  and  double  monsters  in  fishes 

In  his  recent  monograph  Stockard  (192 1)  undertakes 
to  explain  the  morphology  and  physiology  of  twinning 
in  the  fishes,  the  basis  of  his  theory  being  certain  experi- 
ments in  artificial  twin  production  in  Fundulus.  The 
starting-point  of  his  work  was  evidently  my  own  theory 
of  the  causes  of  twinning  in  the  armadillo.  He  also 
adopts  Gemmill's  theory  of  the  origin  of  double  monsters. 
Patterson's  "budding  theory"  of  twin  origin,  and  Child's 
theory  of  dominance  and  subordination. 

In  so  far  as  Stockard's  theory  depends  upon  GemmilPs 
idea  of  origin  of  double  monsters  by  fusion  and  upon 
Patterson's  budding  theory  of  the  origin  of  armadillo 
quadruplets,  his  arguments  and  conclusions  are  unsound. 
Quite  apart  from  the  fact  that  his  theory  has  been 
erected  upon  these  two  insecure  foundations.  Stockard's 
own  extensions  and  applications  of  these  theories  require 
additional  scrutiny. 


62  THE  PHYSIOLOGY  OF  TWINNING 

Starting  with  GemmilFs  idea  that  in  fishes  the 
germ  ring  may  be  regarded  "asa  stock,  able  to  give  rise 
vegetatively,  so  to  speak,  to  more  than  one  embryo,' 
Stockard  builds  up  a  theory  of  twinning  which  I  may 
call  the  " accessory  budding  theory."  The  analogies 
are  drawn  largely  from  the  plant  world.  In  Bryo- 
phyllum,  for  example,  it  is  known  that  the  notches 
around  the  border  of  the  leaf  have  the  power  to  bud 
and  give  rise  to  new  plants.  As  a  rule,  under  ordi- 
nary atmospheric  conditions,  only  one  or  two  notches 
give  off  new  shoots.  The  presence  of  these  shoots  seems 
to  inhibit  the  appearance  of  others.  If,  however,  these 
other  notches  are  isolated,  each  may  produce  a  new 
shoot. 

The  periphery  of  the  blastoderm  in  the  eggs  of  the  bird  and 
mammal  or  the  germ  ring  in  a  teleost's  eggs  is  probably  in  some 

sense  comparable  to  the  notched  border  of  the  budding  leaf 

There  are  many  potential  points  around  the  germ  ring  at  which 
an  embryonic  axis  might  arise.  Here  again,  as  in  the  plant, 
when  one  bud  or  embryonic  axis  has  arisen,  it  tends  to  suppress 
the  potential  ability  of  other  points  to  form  an  axis,  and  normally 
only  one  individual  is  developed  in  the  egg. 

We  are  entirely  unable  to  state  the  reasons  why  a  certain 
point  along  the  germ  ring  should  form  the  bud  and  not  another. 
One  can  only  imagine  that  this  point  has  some  peculiar  advantage 
of  position  which  gives  to  it  a  higher  power  of  oxidation  and  a 
temporarily  more  rapid  rate  of  cell  proliferation  than  is  possessed  by 
other  points,  just  as  the  notch  which  is  dipped  below  the  water  sur- 
face possesses  a  budding  advantage  over  the  other  notches  around 
the  leaf.  Can  the  advantage  of  position  possessed  by  a  particular 
point  of  the  germ  ring  be  reduced  so  as  to  equalize  the  budding 
tendency  of  several  points  and  thus  allow  them  to  express  their 
ability  to  form  embryonic  axes?  Could  such  a  condition  be 
brought  about,  double  embryos,  twins,  triplets,  etc.,  would  be 
produced. 


DOUBLE  MONSTERS  IN  FISHES  63 

The  chief  criticisms  of  this  point  of  view  are  these: 

a)  The  germ  ring  is  not  a  stock  from  which  buds 
appear,  for,  long  before  there  is  any  germ  ring  the  axis 
of  the  single  or  of  twin  embryos  is  already  established. 
The  situation  in  the  bony  fish  is  similar  to  that  in  the 
starfish.  The  embryonic  axis  is  established  at  an  early 
blastoderm  stage,  as  has  been  shown  by  the  analyses  of 
Dr.  Hyman  (192 1).  One  can  readily  determine  that 
one  side  of  the  blastoderm  is  the  posterior  end,  or  the 
end  at  which  gastrulation  will  occur.  There  is  no  such 
thing  as  a  germ  ring  until  long  after  the  blastoderm 
exhibits  definite  axiate  relations.  When  two  embryonic 
shields  arise  they  have  originated  through  a  breaking 
down  of  the  original  polarity  and  a  physiological  isolation 
of  two  regions  of  gastrulation  on  the  blastoderm  before 
any  germ  ring  is  present.  When  the  germ  ring  appears 
it  is  merely  a  non-embryonic  region  of  the  blastoderm, 
concerned  primarily  with  yolk  overgrowth,  and  it  ulti- 
mately forms  merely  the  neck  of  the  yolk  stalk. 

b)  If  the  early  blastoderm,  long  before  germ-ring 
formation,  already  has  a  definite  anterio-posterior  axis 
and  the  point  of  gastrulation  is  well  established,  we  can 
hardly  agree  with  Stockard's  assertion  that  "we  are 
entirely  unable  to  state  the  reasons  why  a  certain  point 
along  the  germ  ring  should  form  the  bud  and  not 
another."  Surely  the  location  of  the  region  of  gastrula- 
tion is  not  due  merely  to  "some  peculiar  advantage  of 
position  which  gives  it  a  higher  power  of  oxidation  and 
a  temporarily  more  rapid  rate  of  cell  proliferation." 
Gastrulation  is  the  same  process  wherever  it  occurs, 
and  it  takes  place  at  points  predetermined  by  the  original 
polarity  of  the  egg.     One  only  need    refer  to  the   ease 


64  THE  PHYSIOLOGY  OF  TWINNING 

of  the  starfish  again  to  make  this  clear.  Under  normal 
conditions  the  point  of  gastrulation  is  exactly  at  the 
vegetal  pole  or  the  posterior  end  of  the  primary  axis  and 
this  point  is  fixed,  in  all  probability,  before  cleavage  begins. 

c)  If  one  wishes  to  stretch  the  concept  of  budding 
so  as  to  make  it  include  the  process  of  gastrulation  it 
might  be  legitimate  to  refer  to  additional  archentera  or 
points  of  gastrulation  as  accessory  buds.  Personally, 
however,  I  see  no  reason  for  viewing  as  a  process  of 
budding  the  ingrowth  of  the  archenteron.  One  might 
equally  readily  consider  any  other  point  of  rapid  growth 
as  a  bud.  If  we  are  unwilling  to  accept  the  term  budding 
for  the  normal  process  of  gastrulation,  it  seems  hardly 
feasible  to  use  this  term  for  additional  centers  of  gastru- 
lation. 

d)  The  question  now  arises  as  to  whether  all  fish 
twins,  as  Gemmill  and  Stockard  believe,  result  from 
separate  points  of  gastrulation  (embryonic  axes).  It 
seems  to  me  highly  probable  that  many,  possibly  all, 
twins  which  are  separate  at  the  two  extremities  and 
united  only  in  the  middle  region  by  external  connections, 
do  actually  arise  from  the  physiological  isolation  of 
separate  embryonic  shields.  If  the  original  polarity  of 
the  blastoderm  be  largely  obliterated  two  secondary 
growing-points  of  equivalent  value,  or  possibly  of  slightly 
different  values,  may  arise,  neither  one  of  which  is  domi- 
nant over  the  other.  From  such  twin  axes  will  develop 
the  types  of  twin  fishes  which  we  have  previously  classed 
as  separate  twins.  If,  however,  the  original  polarity 
is  only  slightly  weakened  so  that  a  secondary  area  of 
gastrulation  arises  at  the  opposite  side  of  the  blastoderm, 
such  an  area  is  likely  to  form  a  small  embryonic  shield 


DOUBLE  MONSTERS  IN  FISHES  65 

which  gives  rise  to  a  smaller,  somewhat  inhibited  embrvn 
that  may  subsequently  come  to  be  hardly  more  than  a 
parasite  on  the  body  of  the  primary  individual.  This 
would  be  my  interpretation  of  most  of  the  cases  of 
parasite  and  autosite  described  by  Stockard  and  others 
(Figs.  25  and  26).  Windle  has  cited  a  case  in  which  the 
parasite  has  been  reduced  to  a  mere  bump  on  the  side 
of  the  autosite.  If  we  were  using  the  terminology  of 
budding  it  would  be  fair  to  consider  such  a  parasite  as 
the  product  of  a  " secondary  bud,"  because  the  primary 
"bud"  has  retained  its  identity.  When,  however,  the 
twin  axes  are  both  new  and  equivalent,  neither  would 
be  a  secondary  "bud."  Though,  as  we  have  seen,  the 
budding  conception  seems  far  fetched  and  valueless,  it  is 
relatively  unobjectionable  when  restricted  to  separate 
twins.  It  is  when  this  concept  is  carried  over  to  the  field 
of  conjoined  twins  that  we  find  it  utterly  untenable. 

THE   LATERAL  BUDDING    THEORY   OF   THE 
ORIGIN   OF    CONJOINED   TWINS 

Stockard  notices,  as  had  many  others  before  him, 
that  the  two  components  of  a  pair  of  conjoined  twins 
are  frequently  of  unequal  size.  As  a  rule  the  larger 
component  is  practically  normal,  while  the  smaller 
exhibits  various  evidences  of  having  suffered  inhibition. 
Such  smaller  components  frequently  show  the  same 
types  of  abnormality  (cyclopia  and  similar  defects)  as  arc- 
seen  in  single  embryos  that  have  been  exposed  at  an  early 
period  to  growth-retarding  agents.  Stockard  interprets 
this  situation  somewhat  as  follows.  The  normal  com- 
ponent is  thought  of  as  arising  from  a  primary  embryonic 
shield  which  would  have  formed  a  single  embryo  but 


66  THE  PHYSIOLOGY  OF  TWINNING 

for  the  interruption  of  development.  The  abnormal 
component  is  viewed  as  the  product  of  a  secondary  or 
lateral  bud.  An  analogy  is  presented  between  this 
condition  and  that  seen  in  certain  plants  with  a  terminal 
growing-point.  So  long  as  the  terminal  growing-point 
(equivalent  to  the  primary  bud  or  the  normal  embryo) 
retains  its  normal  rate  of  growth,  secondary  buds  are 
inhibited;  but  if  the  primary  bud  be  injured  or  removed, 
secondary  buds  (equivalent  to  the  smaller  embryo) 
arise  and  grow,  but  are  often  partially  inhibited  by  the 
presence  of  the  primary  bud.  This  theory  seems  vaguely 
to  imply  that  the  smaller  component  arises  in  some  way 
from  the  side  of  the  primary  axis  like  a  lateral  branch, 
while  the  original  head  remains  intact  as  the  head  of 
the  larger  component.  Possibly,  however,  no  such  crude 
analogy  is  intended,  but  we  are  merely  meant  to  infer 
that  a  smaller  "secondary  bud"  or  embryonic  shield 
arises  on  the  germ  ring  and  that  through  the  process 
of  concrescence  the  primary  and  secondary  individuals 
fuse  in  such  a  fashion  as  to  give  us  individuals  duplex 
anteriorly  and  simplex  posteriorly.  Whichever  of  these 
alternatives  is  meant,  one  is  as  untenable  as  the  other. 
The  lateral  budding  idea  is  quite  incompatible  with  the 
fact  that  even  when  two  components  are  distinctly 
unequal  they  contribute  quite  symmetrically  of  all 
their  median  organs  to  the  single  part  of  the  body. 
Such  an  idea  would  involve  the  assumption  that  budding 
began  internally  so  as  to  involve  notochord,  neural  tube, 
and  all  other  median  structures,  and  that  these  struc- 
tures divided  equally  between  the  stock  and  the  bud — 
a  process  that  would  be  not  budding  at  all  but  fission. 
The  second  alternative  involves  the  old-fashioned  view 


DOUBLE  MONSTERS  IN  FISH  I  -  67 

of  embryo-formation  by  concrescence  and  a  secondary 
fusion  of  originally  separate  embryonic  axes,  a  view 
which  we  have  already  shown  to  be  discredited. 

How  then  can  we  explain  the  apparent  partial  sup- 
pression of  one  component?  Two  simple  explanations 
come  to  mind.  The  first  is  suggested  by  a  study  of  the 
interinnuences  of  human  one-egg  twins  (see  chapter  x). 
The  commonest  mode  of  interinfluence  which  may  be 
detrimental  to  one  or  both  twins  is  shown  to  have  to  do 
with  anastomoses  of  the  placental  blood  vessels.  Now 
Coste,  as  long  ago  as  1855,  showed  that  frequent  anasto- 
moses occur  among  the  vitelline  blood  vessels  of  fish 
twins  and  double  monsters.  Sometimes  pronounced 
inequalities  were  noted  in  the  relative  sizes  of  the 
vitelline  veins  of  the  twins.  The  hearts  of  twins  fre- 
quently beat  alternately  so  that  there  might  occur  back 
pressures  through  the  anastomoses.  These  observations 
on  fish  twins  seem  to  imply  that  the  opportunities  for 
one  twin  to  injure  the  other  through  the  circulation  are 
as  good  as  they  are  in  the  case  of  human  twins;  and  how- 
much  injury  may  be  done  in  the  case  of  the  latter  is 
hereinafter  abundantly  shown. 

The  second  explanation  really  implies  the  adoption 
of  the  fission  theory  of  the  origin  of  double  monsters. 
This  theory  maintains  that  such  conjoined  twins  always 
arise  through  the  separation,  more  or  less  extensive,  of 
the  two  bilateral  halves  of  a  single  embryonic  shield. 
It  is  held  that  the  separation  of  the  two  halves  of  the 
axis  is  due  to  a  lowered  rate  of  metabolism  at  the  time 
when  the  axis  is  being  established,  i.e.,  during  the  time 
just  preceding  gastrulation.  The  primordium  of  one 
half  of  the  axis  may,  after  physiological  isolation  from 


68  THE  PHYSIOLOGY  OF  TWINNING 

the  other,  be  more  severely  inhibited  and  hence  develop 
less  normally  than  the  other.  The  question  arises  as  to 
whether  we  have  any  evidence  that  such  a  physiological 
asymmetry  of  the  bilaterally  equivalent  primordia  exists. 
There  is  in  abundance  exactly  such  evidence.  Anyone 
who  has  engaged  in  the  experimental  production  of 
monsters  in  fishes  cannot  help  but  recall  that  one  of  the 
commonest  types  of  deformity  is  unilateral  in  its  occur- 
rence. One  finds  many  embryos  with  a  normal  and  a 
subnormal  eye,  with  one  pectoral  fin  smaller  than  the 
other.  Another  very  common  type  of  deformity  is 
that  in  which  one  whole  side  has  been  relatively  sup- 
pressed so  as  to  cause  the  animal  to  have  a  curved  or 
spiral  axis.  Now,  if  in  a  single  untwinned  individual 
one  side  may  be  inhibited  while  the  other  remains 
normal,  there  is  no  difficulty  about  explaining  the 
difference  between  the  bilaterally  placed  components  of 
double  monsters.  When  once  one  component  becomes 
relatively  inhibited  it  might  be  secondarily  further 
suppressed  by  the  stronger  twin  through  the  medium 
of  the  circulation  and  might  ultimately  be  almost  or 
quite  obliterated.  Several  instances  have  been  cited, 
both  in  human  twins  and  in  fish  twins,  in  which  such  a 
relatively  inhibited  component  of  a  true  double  monster 
is  seen  to  be  reduced  to  the  condition  approximating 
that  of  a  parasite  on  the  body  of  the  normal  component. 

THE   FISSION   THEORY   OF   THE    ORIGIN   OF 
CONJOINED   TWINS   IN   FISHES 

This  theory  depends  on  the  conception  that  a  bilateral 
organism  is  in  a  sense  a  dual  individual.  A  limited 
amount  of  concrescence  obviously  occurs  even  in  forms 


DOUBLE  MONSTERS  IN  ITS]  \V<  69 

in  which  the  germ  ring  plays  no  part  in  the  formation  of 
the  embryonic  axis.  This  means  that  the  two  bilateral 
primordia  are  formerly  more  or  less  separate  and  that 
at  the  tune  of  gastrulation,  or  the  formation  of  the  axis 
of  symmetry,  there  is  a  highly  energetic  coming  together 
of  the  cells  of  the  two  sides  so  as  to  converge  in  a  median 
dorsal  line.  The  energy  of  concrescence  is  greatest  at 
the  anterior  end,  as  may  be  determined  by  susceptibility 
experiments,  and  progressively  less  great  farther  down 
the  axis.  If  at  the  time  when  the  process  of  concrescence 
is  going  on  most  actively  the  rate  of  metabolism  is 
markedly  lowered,  or  even  if  the  actual  interruption  of 
development  has  been  earlier  and  its  effect  still  persists 
at  the  time  of  gastrulation,  this  process  may  be  more  or 
less  inhibited.  If  the  inhibition  is  slight  it  may  affect 
only  the  most  susceptible  structures  such  as  the  forebrain 
and  the  eyes;  if  a  little  more  severe,  certain  dorsal  or 
median  anterior  structures  may  be  prevented  from 
concrescing;  if  considerably  more  severe,  the  effect  may 
be  felt  far  down  the  axis  and  all  structures  except  certain 
median  ventral  ones  may  be  divided.  This  view  is 
entirely  consistent  with  the  facts.  It  rationalizes  the 
symmetrical  arrangements  of  the  divided  primordia, 
for  if  the  notochords  or  neural  tubes  are  the  products  of 
the  bilateral  fission  of  a  single  primordium,  what  more 
natural  than  that  they  should  be  equal  and  symmetri- 
cal? It  rationalizes  the  observations  that  situs  inversus 
viscerum  is  of-  very  common  occurrence,  for  it  the  two 
components  of  conjoined  twins  are  derived  from  the 
bilateral  primordia  of  a  single  individual  we  might 
expect  to  find  such  evidences  of  mirror-image  symmetry. 
This  view  is  essentially  one  of  the  physiological  isolation 


70  THE  PHYSIOLOGY  OF  TWINNING 

of  bilateral  primordia  through  suppression  of  the  active 
focusing  of  equivalent  bilateral  anlage  upon  a  single 
median  line. 

Cases  of  more  or  less  complete  isolation  may  occur. 
The  minimal  cases  are  probably  those  involving  minor 
degrees  of  discoordination  and  consequent  asymmetry 
of  the  two  sides,  as  in  fish  embryos  in  which  one 
side  grows  and  develops  less  well  than  the  other.  It 
seems  likely  that  the  condition  known  as  hemihyper- 
trophy  in  man  (chapter  xi)  is  to  be  viewed  also  as  a 
case  of  minimal  twinning.  The  maximal  cases  are  those 
in  which  the  two  components  are  entirely  separate  except 
for  a  common  anus  or  a  common  median  ureter.  It  may 
also  be  true  that  some  completely  separate  fish  twins 
arise  by  simply  going  a  short  step  farther  than  the  last- 
named  condition.  Unless  this  is  true  it  would  be  difficult 
to  account  for  the  occasional  cases  of  symmetry  reversal 
in  one  of  a  pair  of  completely  separate  twins.  It  is  my 
opinion,  however,  that  the  great  majority  of  separate 
twins  come  from  separate  embryonic  axes  and  that  all 
true  conjoined  twins  arise  by  the  dichotomy  or  fission  of 
a  single  embryonic  axis. 

THE   EXPERIMENTAL  PRODUCTION   OF 
TWINS  IN  FISHES 

Gernmill  expresses  the  opinion  "that  the  occurrence 
of  double  monstrosity  (twinning)  is  due  in  the  main  not 
to  environmental  factors,  but  to  conditions  which  are 
inherent  in  the  germ  cell."  In  another  connection  he 
says:  "The  likelihood  cannot  be  excluded  that  external 
factors  sometimes  induce  the  production  of  double 
monstrosities  in  the  developing  eggs  of  fishes." 


DOUBLE  MONSTERS  IN  FISHES  71 

Farther  than  a  realization  that  twins  might  be  due 
to  environmental  changes,  Gemmill  did  not  go.  His 
attitude  is  essentially  that  of  a  pure  morphologist  of  the 
old  school  and  not  that  of  an  experimenter. 

Stockard,  however,  taking  his  cue  from  my  own 
theory  of  the  cause  of  twinning  in  the  armadillo,  under- 
took with  some  degree  of  success  to  produce  fish  twins 
by  lowering  the  rate  of  development  of  the  early  embryo. 
He  found  that  by  putting  recently  fertilized  eggs  of 
Fundulus  into  a  refrigerator  for  fairly  long  periods 
and  then  bringing  them  back  to  normal  temperature, 
he  obtained  a  few  double  monsters.  Since  twins  in 
Fundulus  are  extremely  rare  there  can  be  no  doubt  but 
that  twinning  was  induced  by  the  cold.  Stockard  got 
similar  results  in  a  few  instances  with  lack  of  oxygen. 
The  common  factor  was,  as  he  calls  it,  "arrested  develop- 
ment." He  feels  that  there  is  a  very  intimate  relation 
between  twinning  and  the  process  of  gastrulation,  but 
is  rather  vague  about  what  this  relation  is.  In  one  place 
he  says:  "Either  stopping  development  or  greatly 
reducing  its  rate  during  cleavage  or  before  the  germ 
ring  has  formed,  that  is,  at  periods  preceding  gastrulation, 
frequently  serves  to  cause  doubleness  in  the  subsequent 
embryo  formation."  In  another  place  he  writes:  The 
origin  of  two  embryonic  axes  or  growing-points  on  the 
germ  ring  of  the  fish  probably  results  from  a  rather  mild 
or  slight  reduction  in  the  normal  developmental  rate  at 
the  time  of  gastrulation  or  embryonic  shield  formation." 
We  are  thus  left  in  doubt  as  to  the  critical  period  for 
twinning.  Stockard  seems  to  waver  between  two 
positions  and  it  is  uncertain  whether  he  considers  the 
critical  period  before  or  during  gastrulation.      He  has. 


72  THE  PHYSIOLOGY  OF  TWINNING 

however,  rendered  a  distinct  service  to  our  understanding 
of  the  causes  of  twinning  by  his  experimental  demonstra- 
tion that  twins  may  be  produced  experimentally  by 
lowering  the  developmental  rate.  Just  why  some  eggs 
twin  while  others  remain  normal  and  still  others  in  the 
same  batch  exhibit  various  types  of  single  deformity 
we  do  not  at  all  know.  We  have  known  for  a  long  time, 
however,  that  eggs  are  highly  variable  in  their  suscepti- 
bilities to  various  inhibiting  agents.  It  probably  requires 
just  a  certain  degree  of  susceptibility  in  an  egg  and  a 
certain  degree  and  duration  of  inhibition  to  give  the 
particular  result  we  call  twinning;  and  just  the  right 
combination  of  the  two  variables  does  not  often  occur  in 
Fundulus. 

Evidently  in  trout  the  frequency  of  twinning  is 
greater,  but  even  there  the  percentage  of  twins  is  very 
small,  for  there  is  hardly  more  than  one  twin  in  1,000 
eggs  under  the  conditions  prevailing  in  fish  hatcheries. 
Doubtless,  however,  if  Stockard's  methods  were  tried 
on  the  trout  or  the  salmon,  relatively  large  numbers  of 
twins  would  be  produced. 


CHAPTER  VI 
TWINNING  IN  BIRDS 

There  is  a  voluminous  literature  on  twinning  (dupli- 
city) in  birds.  The  egg  of  the  domestic  fowl  has  for 
a  long  time  been  a  classic  object  for  the  study  of  verte- 
brate embryology  and  many  thousands  of  embryos  have 
been  incubated  and  examined  every  year  in  all  parts  of 
the  civilized  world.  Doubtless  every  embryoiogist  who 
has  handled  any  considerable  number  of  chick  embryos 
has  encountered  one  or  more  cases  of  duplicity.  As  a 
result  of  this,  numerous  papers  have  been  published 
about  double  monsters  in  the  chick,  and  a  great  many 
examples  of  various  kinds  of  duplicity  have  been  figured. 

THE   WORK   OF   DARESTE 

The  classic  treatise  on  double  monsters  in  birds  is 
Dareste's  Production  Artificielle  des  Monstruositcs  (1891). 
This  volume  marks  a  new  era  in  the  scientific  study  of 
teratology,  for  the  method  is  thoroughly  experimental. 
By  manipulating  the  temperature,  humidity,  and  oxygen 
conditions  at  various  stages  of  embryonic  development 
he  produced  all  of  the  regulation  types  of  single  monstr<  >s 
ity  and  developed  a  theory  of  teratology  which  has  been 
adopted  and  extended  within  recent  years  by  Stockard. 
He  believed  that  the  great  majority  of  monstrosities  arc- 
developmental  arrests  in  the  sense  that  certain  organs  at 
critical  periods  in  their  development  are  permanently 
halted  and  fail  to  reach  the  normal  definitive  condition. 
Each  organ  or  system  has  some  especially  susceptible 

73 


74  THE  PHYSIOLOGY  OF  TWINNING 

period  when  it  is  readily  arrested.  A  few  anomalies  are 
interpreted  by  Dareste  as  the  result  of  developmental 
excess,  a  sort  of  supernormal  development.  Some  types 
of  monsters  are  interpreted  as  the  result  of  adhesions  and 
of  unions  of  similar  parts. 

The  part  of  Dareste's  treatise  which  especially  inter- 
ests us  at  present  is  that  in  which  he  deals  with  the  origin 
of  double  monsters  and  twins.  He  reviews  the  history 
of  theories  of  the  origin  of  double  monsters,  theories  that 
date  back  as  far  as  the  beginning  of  the  eighteenth  century. 

The  anatomist  Duverney  in  1706  published  an 
account  of  the  organization  of  an  ischiopagus  human 
monster  and  expressed  the  conviction  that  it  could  not 
have  arisen  through  the  partial  fusion  of  two  embryos 
but  must  have  pre-existed  as  a  double  monster  in  the 
egg.  He  was  imbued  with  the  prevalent  doctrine  of 
preformation  and  therefore  found  the  idea  of  a  completely 
preformed  double  monster  more  acceptable  than  one 
involving  epigenetic  changes. 

In  1724  Lemery,  on  the  basis  of  another  dissection 
of  a  two-headed,  single-bodied  human  double  monster, 
sought  to  prove  that  such  monstrosities  could  not  have 
been  preformed  but  must  have  resulted  from  two  separate 
embryos  derived  from  two  eggs. 

Winslow  came  to  the  support  of  Duverney's  position 
and  opposed  the  fusion  idea  on  the  grounds  that  the 
double  mbnsters  were  symmetrically  united  and  that  one 
of  the  components  showed  situs  inversus  viscerurn,  a 
condition  impossible  to  account  for  on  the  basis  of  the 
fusion  theory. 

Wolff  (1772)  combated  the  doctrine  of  preformation 
and   revived   the   epigenesis   doctrine   of   Harvey.     He 


TWINNING  IN  BIRDS  75 

agreed  with  Winslow's  objections  to  the  idea  of  fusion 
and  proposed  the  theory  that  double  monstrosity  was 
determined  through  some  peculiarity  of  the  process  of 
fecundation. 

Meckel,  who  was  a  follower  of  Wolff,  carried  the 
epigenesis  conception  still  farther  and  concluded  that 
all  double  monsters  are  cases  of  developmental  excess, 
inasmuch  as  they  are  derived  from  a  single  egg.  All 
supernumerary  parts  are  conceived  of  as  the  result  of 
a  complete  doubling  of  a  particular  organ.  This  doub- 
ling might  involve  only  one  finger  or  the  whole  body,  as 
in  the  case  of  twins.  He  believed  in  a  dual  origin  of  a 
bilaterally  symmetrical  animal,  and  that  in  double 
monsters  the  two  halves  fail  to  unite  or  unite  only 
partially.  Like  Wolff  he  sought  to  explain  the  doubling 
as  the  result  of  a  peculiarity  of  the  process  of  fecundation 
such  as  double  fertilization  or  some  other  irregularity. 

In  1826  Etienne  Geoff roy  Saint-Hilaire  revived  the 
fusion  idea  of  Lemery,  but  instead  of  supposing  that  the 
fusion  was  a  purely  accidental  phenomenon,  he  tried  to 
explain  the  striking  symmetry  of  monsters  by  stating 
that  homologous  organs  have  an  affinity  for  each  other 
and  only  like  parts  would  fuse  with  like.  Saint-Hilaire 
did  not  follow  up  this  hypothesis  but  passed  it  on  to  his 
son,  who  developed  the  idea  much  farther.  The  latter 
urged  against  the  theory  of  Meckel  such  cases  as  double 
pelvis,  double  breasts,  double  faces,  which,  however, 
seemed  to  him  easily  explicable  on  the  basis  of  the 
hypothesis  of  original  duality. 

So  influential  was  the  fusion  theory  of  the  two 
Saint-Hilaires  that  it  has  been  adopted  by  Dareste,  by 
Gemmill,  and  by  Stockard. 


76  THE  PHYSIOLOGY  OF  TWINNING 

Dareste,  however,  made  a  distinct  advance  in  that 
he  adopted  for  study  the  bird  egg  where,  if  anywhere, 
he  should  have  been  able  to  see  double  monsters  in  the 
making.  He  found  a  number  of  cases  of  two  or  more 
blastoderms  on  a  single  egg;  not  only  that,  but  two  em- 
bryos upon  a  single  blastoderm.  He  noticed  that  these 
paired  embryos  on  one  blastoderm  often  lie  symmetrically 
with  reference  to  each  other  and  this  to  him  seems  to 
make  the  idea  of  double  monsters  as  fusion  products 
entirely  reasonable.  On  these  grounds  he  adopted  the 
theory  that  double  monsters  are  the  product  of  the 
fusion  of  two  embryonic  axes  that  have  arisen  on 
a  single  blastoderm.  He  favors  the  theory  of  the 
Saint-Hilaires  that  like  part  tends  to  fuse  with  like, 
though  there  are  absolutely  no  grounds  for  such  an 
assumption. 

As  to  the  causes  of  twinning  Dareste  has  no  theory. 
One  would  think  that  a  man  so  imbued  with  the  idea 
that  monstrosities  are  all  due  to  developmental  disturb- 
ances might  readily  have  concluded  that  double  monsters 
were  so  produced,  but  he  did  not  seem  to  take  this  step. 
On  the  contrary,  he  specifically  states  that  twins  and 
double  monstrosities  are  predetermined  before  laying 
through  some  disturbance  of  the  process  of  fertilization. 
For  him  double  monsters  are  not  in  the  same  category 
with  the  numerous  types  of  single  monstrosity  which  he 
describes  so  much  in  detail. 

MODES   OF   TWINNING   IN   BIRDS 

Since  the  appearance  of  Dareste's  treatise  a  large 
literature  concerning  double  monsters  in  birds  has  grown 
up,  and  many  excellent  figures  of  double  monsters  of 


TWINNING  IN  BIRDS 


77 


many  kinds  have  been  published.  A  survey  of  a  lar 
number  of  these  figures  and  of  sections  through  crucial 
parts  of  double  embryos,  together  with  a  study  of  a 
collection  of  several  pairs  of  chick  twins  in  my  own 
possession,  has  convinced  me  that  the  situation  is 
essentially  the  same  as  that  described  for  the  star- 
fish; that  the  modes  of  twinning  and  the  different 
categories  of  twins  are  essentially  the  same  in  two 
cases.  I  find  in  the  birds  the  following  kinds  of  twins: 
(i)  those  derived  from  two  blastoderms  on  a  single 
yolk;  (2)  those  derived  from  two  embryonic  axes  on 
one  blastoderm;  (3)  those  derived  by  the  isolation, 
more  or  less  complete,  of  the  bilateral  halves  of  a  single 
embryonic  axis;  (4)  those  derived  by  the  growing 
together  of  two  separate  embryonic  axes;  and  (5)  those 
in  which  one  compo- 
nent of  a  double  mon- 
ster is  more  or  less 
completely  absorbed 
by  the  other.  These 
five  categories  of 
twins  will  be  discussed 
seriatim. 

1.  That  separate 
blastoderms  on  a 
single  yolk  actually 
do  occur  seems  cer- 
tain in  view  of  the 
clear  figures  of  Da- 
reste  and  of  Kaestner. 
That  of  Dareste 
(Fig.  34)  represents  a 


Fig.  34. — A  case  of  triplets  in  which 
the  upper  individual  probably  arose  from 
an  early  fission  of  the  blastoderm  into  two 
unequal  masses.  The  lower  pair  of  twins 
is  obviously  the  result  of  double  gastru- 
lation  of  the  Larger  moiety  of  the  divided 
blastoderm.     'After  I  >arest 


78 


THE  PHYSIOLOGY  OF  TWINNING 


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case  of  triplets,  two  embryos  derived  from  one  blasto- 
derm and  one  from  the  other.     The  twins,  seen  in  the 

lower  part  of  the  figure 
are  believed  to  have 
been  derived  from  one 
blastoderm  as  the  result 
of  double  gastrulation. 
Though  these  two  em- 
bryos are  in  all  proba- 
bility the  result  of  two 
secondary  areas  of  gas- 
trulation and  are  there- 
fore bilateral  equiva- 
lents, one  is  considerably 
more  advanced  than  the 
other.     Such   examples 


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Fig.35— An  early  stage  of  twinning    of   unilateral   asymme- 

in  the  chick,  evidently  the  product  of  try  are  no  more  difficult 
an  unequal  fission  of  an  early  blasto-  of  eXplanation  than  are 
derm.     (After  Kaestner.)  .  .  .  . 

cases  in  which  one  side 
of  a  single  embryo  develops  more  rapidly  than  the  other. 
A  clearer  case  of  twin  chicks  derived  from  two  separate 
blastoderms  is  one  presented  by  Kaestner  (Fig.  35). 
This  case  consists  of  a  pair  of  twins  in  the  primitive 
streak  stage.  Here  we  have  a  good  example  of  a  blasto- 
derm which  has  undergone  bilateral  fission  so  as  to  form 
two  rather  unequal  blastoderms.  The  fact  that  the 
two  embryos,  though  of  very  unequal  size,  bear  a 
mirror-image  relation  to  each  other,  is  significant. 

2.  Separate  twins  arising  from  two  points  of  gastrula- 
tion on  a  single  blastoderm  are  usually  symmetrically 
placed  so  that  the  anterior  ends  of  the  axes  tend  to 


TWINNING  IN  BIRDS  79 

approach  each  other  toward  the  center  of  the  blasto- 
derm. Sometimes  the  two  embryos  remain  entirely 
separate,  at  least  for  a  time.  Kaestner  (1901)  figures 
a  twin  chick  in  the  primitive  streak  stage  in  which 
the  two  axes  were  in  the  same  line,  much  like  the  twin 
starfish  shown  in  Figure  6,  but  the  anterior  ends  are  still 
some  distance  apart.  The  same  author  presents  a  very 
clear  photograph  (Fig.  36)  of  a  similar  pair  of  twin 
embryos  in  a  thirteen-  or  four  teen-somite  stage,  with 


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Fig.  36. — A  rare  type  of  chick  twins  that  are  the  product  of  double, 
symmetrical  gastrulation.  The  two  embryonic  axes  have  grown  inward 
toward  each  other  and  have  barely  avoided  a  collision  such  as  has  taken 
place  in  the  twins  shown  in  Fig.  39.     (After  Kaestner.) 

axes  almost  in  the  same  line.  Their  heads  have,  how- 
ever, shoved  past  each  other  as  far  as  the  hind-brains. 
Though  there  is  some  contact  and  confusion  in  the 
membranes,  especially  the  anuria,  of  these  twins,  t hex- 
are  entirely  separate  and  have,  in  my  opinion,  arisen  as 
two  quite  distinct  twin  areas  of  gastrulation  on  opposite 
sides  of  the  blastoderm.  This  opinion  is  strengthened 
by  a  study  of  a  very  interesting  double  monster  in  my 
possession  (Fig.  37).     In  this  case  it  is  obvious,  I  belies 


8o 


THE  PHYSIOLOGY  OF  TWINNING 


that  the  two  inwardly  growing  embryos  have  met  each 
other  head-on  instead  of  growing  past  each  other  as  in 
Kaestner's  case.  On  this  account  there  is  a  considerable 
amount  of  crumpling  up  of  the  brains  and  other  anterior 
parts.  The  heart  of  one  has  been  crowded  off  to  the 
left  and  that  of  the  other,  to  the  right.  Such  a  duplicity 
as  this  cannot  have  arisen  as  a  fission  product  of  a  single 


Fig.  37. — The  result  of  a  head-on  collision  between  chick  twins 
that  have  resulted  from  double,  symmetrical  gastrulation  on  opposite 
sides  of  the  blastoderm.  The  results  of  the  collision  are  seen  in  the 
crumpled  forebrain  and  displaced  hearts.  This  case  is  equivalent  in 
mode  of  origin  to  the  type  of  Patiria  twin  shown  in  Fig.  7.     (Original.) 


embryonic  axis.  Dareste,  Kaestner,  and  Tannreuther 
have  described  cases  similar  to  this,  but  such  purely 
mechanical  fusions  as  this  are  quite  different  in  charac- 
ter from  most  double  monsters,  and  must  not  be  con- 
sidered as  supporting  the  theory  that  cosmobia  (truly 
symmetrical  double  monsters)  are  the  product  of  the 
symmetrical  fusion  of  paired  embryonic  axes. 

A  really  crucial  case  has  recently  come  into  my  hands 
through  the  kindness  of  Dr.  B.  H.  Willier.     This  is  a 


TWINNING  IN  BIRDS 


81 


double-monster  chick  (Fig.  38)  in  which  there  is  appar- 
ently a  fusion  in  the  head  region.     The  two  bodies  are 
very  symmetrically  placed  and  practically  in  the  same 
developmental  stage  (about  seven  somites).     As  com 
pared  with  normal  single  embryos  of  this  age  we  note 


•  ■ 


Fig.  38. — A  practically  symmetrical  pair  of  secondarily  fused  chick 
twins,  resulting  from  a  collision  at  a  tangent  instead  of  head-on  as  in 
Fig.  37.  It  is  such  cases  as  this  that  have  convinced  some  vmters  that 
true  katadidymi  are  the  result  of  the  symmetrical  fusion  of  separate 
embryonic  axes.     (Original.) 

decided  differences.  Though  the  brains  of  the  two 
components  are  fairly  symmetrical,  there  is  obvious 
crumpling  of  both.  No  part  of  a  normal  single  head 
could  possibly  arise  from  such  a  condition.  No  omphalo- 
mesenteric veins  are  present  on  either  component,  while 
the  anterior  vitelline  vein  of  one  embryo  has  developed 
to  one  side  and  that  of  the  other  to  the  other  side.  This 
condition  also  could  never  so  regulate  itself  as  to  give 


82 


THE  PHYSIOLOGY  OF  TWINNING 


a  normal  single  condition.  We  may  conclude  then 
that  true  double  monsters  (cosmobid)  cannot  be  derived 
through  the  coming  together  at  their  anterior  ends  of  separate 
embryonic  axes. 

In  this  category  of  twins  we  may  include  the  very 
rare  case  of  Dareste  (Fig.  39)  consisting  of  a  set  of  triplets 

upon  a  single  blasto- 
derm.  The  facts 
that  the  heads  all 
point  inward  to- 
ward the  center  and 
that  each  lies  on  its 
left  side  like  a  nor- 
mal single  embryo, 
argues  for  their  deri- 
vation from  three 
separate  and  equiv- 
alent points  of 
gastrulation.  No 
other  similar  condi- 
Fig.  39. — A  fine  example  of  identical    tion  has  ever  been 

triplet   chick   embryos   derived  from  triple     described 
gastrulation  of  a  single  blastoderm.     (After  .    ,1*1 

Dareste.)  3-  A  third  type 

of  twin  chick  em- 
bryo is  that  which  has  obviously  arisen  through  the 
separation  of  the  right  and  left  primordia  of  a  single 
embryonic  axis.  Embryos  of  this  sort  are  not  at  all 
uncommon.  One  case  of  Tannreuther's  that  seems  to 
me  to  bear  this  interpretation  is  that  of  a  three-somite 
stage  in  which  there  are  two  entirely  separate  heads 
and  the  body,  back  to  the  posterior  end  of  what  was 
originally  the  head  process,  is  symmetrically  divided  into 


TWINNING  IN  BIRDS  83 

two  equivalent  primordia  (Fig.  40).  The  three  meso- 
blastic  somites  in  the  middle  seem  to  belong  in  common 
to  the  two  embryos.  The  whole  primitive  streak  region 
seems  to  be  single.  In  brief,  the  head-process  region 
back  to  the  primitive  knot  has  twinned  and  the  post- 
cephalic  region  has  remained  untwinned.  Such  dupli- 
cities are  technically  called 
anadidymi. 

As  is  the  case  in  human 
double  monsters   there  is  a  )v 

class  of  chick  duplicities  in  y    / 

which  the  head  region  is  more    i  fM"]*» 

or  less  completely  single  while 
there  are  two  widely  diver- 
ging trunks.  A  typical  ex- 
ample of  this  sort  of  mon- 
ster,  really  a  katadidymus,  is 
figured  by  Tannreuther 
(Fig.  41,  p.  84).  The  head 
region  back  to  the  somites 

tig.   40. — A    typical    douhU- 

is  normal  and  single;    pos-    headed  chick  monster  (anadidy- 

terior  to  that  there   are   two     mus)   resulting  from  the  partial 

bodies  diverging  at  an  angle    dichotomy  of  the  anterior  end  of 
.    .  0      _  .  the  originallv  single   embryonic 

of  about  120.     Tannreuther    axis     (After  Tannreuther.) 

considers  that  this  "  double 

embryo  no  doubt  began  its  development  as  two  inde- 
pendent primitive  streaks,  with  a  later  connection  or 
fusion  of  the  anterior  ends  of  the  two-head  processes." 
With  this  conclusion  I  cannot  agree.  It  seems  to 
me  inconceivable  that  two  separate  axes  could  come 
together  with  such  perfect  symmetry  as  to  form 
a  normal  single  head.     In  brief,  the  objections  t»>  the 


84 


THE  PHYSIOLOGY  OF  TWINNING 


fusion  idea  are  the  same  in  this  case  as  in  other  cases  pre- 
viously considered.  The  condition  is,  in  my  opinion, 
unquestionably  a  result  of  a  process  of  fission  of  the 
posterior  growing  region  of  the  embryo.     It  must  be 


SlOSiVS 


...  •. 

"■-  ■  t..'. 


&/-.:  ■■:<-.:■-  ■■'  -.-; .: 

- 


m 


w 


\a\ 


- 


ft 


'/:-; 


:'\ 


Fig.  41. — A  typical  two- tailed  chick  double  monster  (katadidymus) 
due  to  the  partial  fission  of  the  posterior  end  of  a  single  embryonic  axis. 
(After  Tannreuther.) 

supposed  that  at  the  time  when  the  agencies  responsible 
for  twinning  were  applied,  the  head  parts  of  the  sym- 
metry axis  had  been  definitely  established  and  that  there 
was  a  secondary  point  of  great  activity  and  high  suscep- 
tibility at  the  level  of  the  dorsal  lip  of  the  blastopore  which 
is  probably  at  the  anterior  end  of  the  primitive  streak. 

In  the  chick  there  appear  to  be  really  two  distinct 
regions  of  differentiation  and  two  gradients:  one  the 
head  process  and  the  other  the  primitive  streak.     The 


TWINNING  IN  BIRDS  85 

latter  is  believed  to  be  the  equivalent  of  the  closed  blasto- 
pore and  its  anterior  end  is  the  dorsal  lip  region,  equiv- 
alent to  that  described  as  a  secondary  point  of  high 
susceptibility  in  the  frog.  It  appears  that  either  one  of 
these  regions  may  undergo  fission  without  the  other  or 
that  both  may  undergo  fission  more  or  less  completely. 
Kaestner  (1898)  describes  and  figures  an  interesting 
case  of  a  double  chick  embryo  in  approximately  the  same 
stage  as  that  of  Tannreuther  shown  in  Figure  41,  but 
the  head,  instead  of  being  a  normal  single  region,  is 
much  broader  than  usual  and  is  partially  double,  the 
forebrain  being  single  and  the  rest  of  the  central  nervous 
system  being  double.  Such  a  condition  is  interpreted 
as  a  case  of  partial  fission  of  the  head-process  region  of 
the  blastoderm.  Whether  it  is  possible  for  two  entirely 
separate  chick  embryos  to  arise  through  the  fission  of 
the  bilateral  primordia  of  a  single  embryonic  axis  is  a 
question  at  present  unsettled.  Tannreuther  shows  an 
example  of  twin  chicks  that  I  am  compelled  to  interpret  as 
products  of  a  nearly  complete  fission.  As  may  readily  be 
seen  (Fig.  42,  p.  86)  the  two  embryos  are  entirely  separate 
except  at  the  very  posterior  end  where,  back  of  the  tail- 
bud  region,  the  two  primitive  streaks  unite  for  a  short 
distance.  Unless  some  sort  of  gross  concrescence  of  the 
posterior  margin  of  the  blastoderm  has  occurred,  this 
single  region  of  the  primitive  streak  can  be  explained 
only  as  the  residue  of  an  originally  single  embryonic  a 
that  has  undergone  almost  complete  bilateral  fission.  In 
this  blastoderm  the  two  embryos  are  markedly  different 
in  size,  the  right-hand  one  being  much  larger  and  more 
advanced  (fourteen  somites),  while  the  smaller  one  is  in 
a  ten-somite  stage.     Tannreuther  is  of  the  opinion,  and 


86 


THE  PHYSIOLOGY  OF  TWINNING 


in  this  I  agree,  that  in  this  case  "two  distinctly  inde- 
pendent chick  embryos  would  have  resulted  at  the  end 
of  the  incubation  period." 


^aa^**^ 


Fig.  42. — A  rare  type  of  chick  duplicity,  probably  the  result  of  a 
nearly  complete  longitudinal  fission  of  an  originally  single  embryonic 
axis.  There  are  no  evidences  of  plural  gastrulation  but,  on  the  contrary, 
the  posterior  end  of  the  axis  is  still  single.     (After  Tannreuther.) 

4.  Already  under  the  second  heading  I  have  indicated 
that  there  is  some  very  good  evidence  that  twin  embryos 
originating  from  separate  embryonic  axes  and  arising 
directly  opposite  to  each  other  on  the  same  blastoderm, 
may  grow  toward  each  other,  meet  head  on,  and  fuse  at 
their  anterior  ends  (Figs.  37  and  38).  I  see  no  other  satis- 
factory explanation  of  the  crumpled  condition  of  the  two 
anterior  ends.    Examples  of  such  a  condition  have  already 


TWINNING  IN  BIRDS 


87 


been  described  on  pages  79-81.  In  these  embryos  the 
brains  are  much  folded  and  wrinkled  as  though  each  had 
interfered  with  the  other's  growth. 

5.  Many  authors  have  described  advanced  twin 
embryos  in  which  the  autosite  and  parasite  condition  is 
obvious.  In  my  opinion  this  condition  may  arise  from 
either  of  the  two  categories  numbered  (2)  and  (3),  but 
is  much  more  likely  to  arise  from  (2).  An  unusually 
interesting  case  (Fig.  43)  of  autosite  and  parasite  in  the 
making  is  one  in  my  own  possession.     This  curious  pair 


■ 

I 

N 

'( 


a 
s  1 

'•7    ' 

i 

.  ■ 


Fig.  43. — A  very  unusual  type  of  chick  twin  embryo,  doubtless  .1 
case  that  would  lead  to  the  autosite-parasite  condition.  The  I 
embryos  probably  arose  from  two  separate  points  of  gastrulation,  one 
of  which  was  primary  and  the  other  secondary.  The  smaller,  secondary, 
axis  has  evidently  been  partially  inhibited  by  the  larger,  primary,  axis, 
and  was  destined  to  be  a  mere  parasite  on  the  body  of  the  Latter. 
(Original.) 


88  THE  PHYSIOLOGY  OF  TWINNING 

of  chick  twins  are  of  very  unequal  size.  The  large 
embryo  is  quite  normal  and  has  seventeen  somites.  The 
small  embryo,  which  is  quite  separate  as  yet  from  the 
large  one,  is  in  about  a  four-somite  stage  and  is  therefore 
some  eighteen  hours  less  advanced.  The  head  end  of  the 
small  embryo  nearly  touches  the  body  of  the  large  one 
and  its  axis  is  at  right  angles  to  the  latter.  The  vitelline 
circulation  of  the  large  embryo  is  seen  to  be  invading  the 
vitelline  area  of  the  small  one  and  it  is  highly  probable 
that  the  small  embryo  would  later  have  become  a  mere 
cyst  on  the  side  of  the  larger  or  might  have  been 
totally  absorbed.  If  one  were  to  attempt  to  assign  this 
double  embryo  to  one  of  the  above-mentioned  categories 
he  would  be  compelled  to  select  number  2,  for  there  is 
no  evidence  that  the  two  are  derived  from  separate 
blastoderms.  These  twins  are,  therefore,  the  product  of 
unequal  double  gastrulation. 

THE   CAUSES   OF  TWINNING  IN  BIRDS 

Stockard  points  out  that: 

The  eggs  of  birds  normally  have  a  discontinuous  mode  of 
development.  Fertilization  takes  place  in  the  upper  part  of  the 
oviduct  and  the  egg  begins  its  development  in  the  high  temperature 
of  the  maternal  body  and  continues  to  develop  as  it  travels  down 
the  uterine  tube  and  becomes  surrounded  by  its  accessory  coats. 
Finally  at  the  time  of  laying,  the  blastoderm  has  passed  the 
gastrula  stage.  The  fall  in  temperature  experienced  on  leaving 
the  body  of  the  mother  causes  development  to  stop  in  this  early 
post-gastrula  condition,  and  the  egg  remains  quiescent  until  the 
temperature  is  again  raised  to  about  that  of  the  bird's  body. 

It  is  believed  that  the  reason  why  the  sudden  and 
rather  prolonged  interruption  of  development  so  seldom 
results  in  twinning  is  that  the  critical  period  for  twin- 


TWINNING  IN  BIRDS  89 

ning,  the  period  of  gastrulation,  has  passed  in  most  of 
the  eggs  before  they  are  laid.  Only  a  few  eggs  that 
are  laid  prematurely,  before  the  process  of  gastrula- 
tion has  been  completed,  undergo  twinning.  Patterson 
(1909)  has  shown  that  there  is  considerable  variability 
in  the  state  of  advancement  of  eggs  at  the  time  of 
laying.  Certain  hens  have  a  greater  tendency  than 
others  to  deposit  eggs  before  gastrulation  is  complete. 
If  this  trait  is  hereditary  it  would  not  make  much  prog- 
ress in  the  race  because  the  prematurely  laid  eggs  arc- 
very  likely  to  develop  non-viable  double  monsters  or 
other  abnormalities.  By  a  process  of  natural  selection 
it  has  probably  resulted  that  only  strains  of  birds  with 
an  inherited  tendency  to  lay  the  eggs  relatively  late  have 
survived. 

Although  we  have  no  definite  data  on  the  artificial 
production  of  twins  in  birds  the  prevalence  of  twins 
reared  under  artificial  conditions  implies  that  the  twin- 
ning process  is  the  result  of  some  interruption  of  the 
normal  course  of  embryonic  development  that  results 
in  the  partial  deaxiation  of  the  blastoderm  and  thus 
permits  of  double  gastrulation  or  else  causes  the  physi- 
ological isolation  of  the  bilateral  primordia  of  a  single 
embryonic  axis.  The  fact  that  the  vast  majority  of  bird 
twins  are  .of  the  conjoined  rather  than  of  the  separate 
type  leads  to  the  conviction  that  the  former  arc  as  a  rule 
the  product  of  a  relatively  late  developmental  interrup- 
tion involving  the  final  steps  in  the  establishment  of 
the  embryonic  axis.  An  earlier  retardation  would  be 
expected  to  cause  the  fission  of  the  head  proi  ind  a 
later  retardation  that  of  the  primitive  streak  or  posterior 
parts.     This  would  seem  to  be  a  rational  explanation  oi 


go  THE  PHYSIOLOGY  OF  TWINNING 

the  relatively  marked  prevalence  of  katadidymi  among 
the  birds,  for  this  is  the  commonest  expression  of  twin- 
ning in  the  group,  in  striking  contrast  with  the  situation 
in  the  fishes  where  anadidymi  are  the  almost  universal 
type  and  katadidymi  extremely  rare. 

All  of  these  facts  accord  with  the  phenomenon  pre- 
viously referred  to:  that  in  the  birds  developmental 
interruption  at  the  time  of  laying  occurs  only  after  the 
process  of  gastrulation  has  made  considerable  progress. 
We  would  expect,  then,  and  actually  do  find  that  twins 
from  separate  blastoderms  are  very  infrequent,  that 
those  derived  from  two  gastrulations  are  next  in  rarity, 
that  anadidymi  come  next,  and  that  katadidymi  are  the 
most  frequent. 


CHAPTER  VII 

TWINNING  IN  AMPHIBIA.  REPTILES,  WD 
OTHER  CHORDATES 

AMPHIBIA 

Twinning  appears  to  occur  with  extreme  rarity  among 
the  Amphibia.  The  fact  that  normal  development  takes 
place  at  very  low  temperatures  may  partially  explain  the 
failure  to  twin;  for  low  temperature  seems  to  be  the  main 
reason  for  twinning  among  egg-laying  vertebrates  other 
than  the  fishes.  Twinning  may  readily  be  induced  among 
the  Amphibia,  however,  by  experimental  procedures. 

O.  Schultze  (1894)  showed  in  the  case  of  the  frog 
that  if  the  Qgg  is  inverted  while  it  is  in  the  two-cell  stage 
of  cleavage  in  such  a  way  that  the  white  or  vegetative 
pole  is  turned  upward,  each  blastomere  will  give  rise  to 
a  whole  embryo.  Figure  44,  page  92,  shows  several  of 
his  double  embryos  at  various  stages  of  development. 
Morgan  (1901)  in  commenting  upon  these  results  says: 
"In  this  case  it  appears  that  the  results  are  due  to  a  rota 
tion  of  the  contents  of  each  blastomere  so  that  like  parts 
of  the  two  bias tomeres  become  separated."  There  isals 
doubtless  another  reason  for  the  physiological  isolation 
of  the  two  hemispheres,  viz.,  retardation,  due  to  tin-  long 
delay  consequent  upon  the  necessity  lor  each  blastomere 
to  undergo  a  complete  reorganization  of  its  axiate  struc- 
ture. Each  cell,  after  the  temporary  cessation  of  devel 
opmental  activity,  starts  out  for  itself  as  though  it  weir 
a  separate  egg,  and  each  produces  a  whole  embryo. 

91 


Q2 


THE  PHYSIOLOGY  OF  TWINNING 


Tornier  has  performed  a  number  of  experiments  on 
the  salamander,  Triton,  involving  artificial  doubling  of 
tails  and  limbs,  using  the  methods  of  constriction  and 
cutting  off  of  parts.  This  mechanical  isolation  of  grow- 
ing regions  results  in  the  same  types  of  doubling  of  limbs 
and  tails  as  does  physiological  isolation  in  other  forms. 


Fig.  44.— Types  of  double  frog  embryos,  due  to  inverting  the  eggs 
in  the  two-cell  stage.     (After  Schultz.) 

Spemann  and  Falkenberg  (1920)  have  recently  pre- 
sented some  very  interesting  data  concerning  the  arti- 
ficial production  of  twins  in  Triton.  Early  gastrula 
stages  were  cut  in  two  along  the  sagittal  plane  so  as  to 
make  two  equivalent  right-  and  left-hand  half-embryos. 
These  as  a  rule  regenerated  the  lost  half,  giving  rise  to 
artificial  monozygotic  twins.  About  half  of  the  right- 
hand  pieces  showed  situs  inversus  viscerum,  or  reversed 
symmetry  of  the  heart  and  stomach.     This  interesting 


AMPHIBIA,  REPTILES,  OTHER  CHORDATES       93 

case  of  physical  isolation  of  the  equivalent  halves  of  a 
single  embryonic  axis  closely  parallels  what  we  believe 
occurs  through  physiological  isolation  in  other  group-. 
Inasmuch  as  this  work  is  to  be  discussed  in  a  subsequent 
chapter  dealing  with  the  matter  of  reversed  symmetry, 
we  shall  omit  further  details  in  this  place. 

Bellamy  (1918),  in  his  elaborate  series  of  experiments 
dealing  with  the  modification  and  control  of  development 
in  the  frog,  obtained  one  double-headed  individual  and 
not  a  few  with  single  head  and  more  or  less  completely 
double  bodies  and  tails.  Some  of  the  individuals  were 
double  as  far  forward  as  the  forebrain;  others  were  double 
only  in  the  tail  region.  This  writer  was,  at  the  time  of 
his  experiments,  not  much  interested  in  twinning  and  did 
not  discuss  the  matter  at  any  length  in  his  paper.  He 
merely  says  that  "  spina  bifida  of  all  degrees  is  of  course 
common  under  conditions  that  inhibit  development, 
and  result  primarily  from  inhibition  of  the  dorsal  lip 
region"  of  the  blastopore.  Bellamy  thus  takes  a  posi- 
tion, in  harmony  with  my  own,  that  these  duplicities 
which  he  has  merely  referred  to  as  "spina  bifida'  are 
cases  of  bilateral  twinning  and  are  due  to  the  inhibition 
of  a  particularly  susceptible  region  of  the  embryonic 
axis,  the  dorsal  lip  of  the  blastopore.  If  this  region  of 
growth  is  inhibited  and  lateral  regions  continue  to  grow, 
twinning  is  inevitable.  It  may  be  said  that  the  dorsal 
lip  region  is  probably  a  near  equivalent  of  the  region 
of  the  fish  embryonic  shield  called  by  Kopsch  the  Knopf, 
which  is  the  primordium  of  the  tail-bud.  We  have 
already  shown  that  the  tail-bud  region  is  a  secondarily 
acquired  point  of  high  metabolic  rate  and  very  active 
growth.     Such  a  region  might  readily  be  so  inhibited  as 


94  THE  PHYSIOLOGY  OF  TWINNING 

to  bring  about  the  physiological  isolation  of  two  points 
somewhat  separated  from  the  median  point — that  point 
most  strongly  inhibited — and  the  consequent  production 
of  two  potentially  equivalent  tail-buds. 

An  interesting  case  of  minimal  twinning,  similar  to 
that  described  for  sea-urchin  and  starfish  larvae  in  our 
chapter  on  symmetry  reversal,  is  described  by  Bateson 
(1894)  for  a  tadpole  of  Pelobates  fuscus.  Instead  of 
having  one  spiracle  on  the  left  side,  as  is  normal  for 
Amphibia,  there  are  paired  spiracles  of  equal  size.  This 
is  cited  by  Bateson  as  a  case  of  homoeosis. 

REPTILIA 

Partial  twinning  involving  a  more  or  less  complete 
doubling  of  anterior  structures  is  probably  fairly  common 
among  reptiles;  but  relatively  few  workers  have  given 
attention  to  the  subject  of  reptilian  embryology  and 
therefore  few  cases  of  twins  or  double  monsters  have  been 
reported.  Reptilian  development  is  so  similar  to  that 
of  birds  that  this  field  of  embryology  has  been  relatively 
neglected.  That  twinning  does  occur  probably  with 
even  greater  frequency  and  with  more  success  in  the 
reptiles  than  in  the  birds,  I  have  many  reasons  for  believ- 
ing. While,  as  Bateson  points  out,  there  are  no  authen- 
tic records  of  a  double  monster  in  mammals  or  in  birds 
having  grown  up  in  a  wild  state,  there  are  many  such 
cases  among  the  reptiles.  Several  of  the  older  and  some 
of  the  newer,  more  critical  writers  have  described 
instances  of  complete  or  partial  duplicity  among  the 
snakes,  involving  mostly  double  or  triple  heads.  At 
least  two  authentic  cases  are  on  record.  In  some 
instances  the  doubling  involves  only  the  most  anterior 


AMPHIBIA,  REPTILES,  OTHER  CHORDA  I  I  95 


median  organs,  such  as  the  nostrils.  Instances  are 
known  of  the  occurrence  of  four  nostrils,  representing  an 
even  slighter  degree  of  twinning  than  the  presence  of  a 
third  or  median  eye  in  fishes. 

Gemmill  (191 2)  cites  a 
case  of  a  three-headed  snake 
seen  near  Lake  Ontario  by 
Bruch.  He  also  notes  that 
another  three-headed  snake 
was  reported  by  Andro- 
vandus  from  the  Pyrenees 
Mountains. 

Among  Chelonia  there 
are  very  few  recorded  in- 
stances of  twinning.  Bate- 
son  describes  and  figures  an 
interesting  specimen  of  two- 
headed  tortoise  (Fig.  45)  in 
which  the  heads  behaved  independently  as  though  they 
had  distinct  individualities.  Such  a  creature  might  have 
some  difficulty  in  deciding  on  a  direction  of  locomotion. 
This  is,  so  far  as  I  know,  the  only  recorded  case  of  twin- 
ning among  the  turtles,  but  I  am  convinced  that  embry- 
onic twinning  is  not  infrequent.  While  living  for  some 
years  on  Lake  Maxinkuckee  in  Indiana,  a  lake  plentifully 
stocked  with  several  species  of  turtles,  I  took  occasion 
to  study  this  group  in  a  rather  intensive  fashion.  Among 
other  things  studied  were  the  breeding  and  nesting  habits 
and  the  general  features  of  the  embryology.  Many  a 
morning  expedition  was  made  to  watch  the  turtles  building 
their  nests  in  the  sand  and  in  soil  of  various  characters. 
Most  of  the  species  dig  out  with  their  hind  feet  a  vertical 


Fig.  45. — Double  monster 
turtle  of  the  anadidymus  type. 
I  Alter  Bateson.) 


96  THE  PHYSIOLOGY  OF  TWINNING 

tunnel  several  inches  deep,  enlarged  into  a  flask-like 
chamber  at  the  bottom.  The  incubation  of  the  eggs 
depends  entirely  upon  the  heat  of  the  sun  that  may 
penetrate  the  soil.  The  breeding  season  comes  either  in 
May  or  early  June  and  it  is  sometimes  decidedly  cold 
and  sunless  during  a  considerable  part  of  the  period  of 
incubation.  There  can  be  no  doubt  that  climatic  irregu- 
larities have  a  very  marked  effect  upon  the  development 
of  chelonian  eggs  in  north  temperate  latitudes.  I  have 
examined  a  great  many  nests  and  have  found  whole 
batches  of  eggs  dead  and  decaying,  probably  killed  by  a 
cold  spell  during  the  early  periods  of  incubation.  In 
other  batches  of  eggs  I  have  found  a  very  large  percentage 
of  embryos  abnormal  in  various  respects:  some  with 
imperfect  eyes;  some  with  heads  small  and  irregular; 
some  with  one  or  more  feet  lacking  or  tail  lacking;  some 
with  deformed  carapace;  many  with  irregularities  of  the 
scute  pattern  of  both  carapace  and  plastron,  and  associ- 
ated abnormalities.  All  of  these  irregularities  are 
obviously  due  to  unfortunate  developmental  condi- 
tions— probably  low  temperatures.  In  several  hundred 
embryos  of  various  species  of  turtles  examined  I  have 
never  found  a  case  of  twins  or  even  of  unmistakable 
double  monstrosity.  One  type  of  abnormality,  however, 
that  was  fairly  common  was  a  condition  of  more  or  less 
extensive  doubling  of  the  median  series  of  scutes  on  the 
carapace.  This  type  of  irregularity  was  found  to  be 
closely  correlated  with  a  similar  doubling  of  neural 
plates,  which  are  the  broadened  dorsal  spines  of  the 
vertebrae.  Not  infrequently  there  was  a  dichotomous 
fission  of  a  rib  in  association  with  such  doubling,  and 
when  there  are  twinned  ribs  there  are  usually  twinned 


AMPHIBIA,  REPTILES,  OTHER  CHORDATES       97 

costal  plates  and  accessory  costal  scutes.  In  view  of 
what  we  now  know  about  twinning  in  vertebrates,  I  am 
convinced  that  this  strong  tendency  to  form  a  double 
median  series  of  scutes  and  plates  in  these  subnormal 
turtle  embryos  is  a  case  of  incipient  twinning,  due  to 
partial  isolation  of  the  median  dorsal  elements  of  the 
right-  and  left-hand  primordia  of  the  axis.  That  the 
twinning  process  sometimes  goes  much  farther  than  this 
is  evidenced  by  the  fact  that  two-headed  conjoined 
twins,  such  as  that  shown  in  Figure  45,  actually  occur. 

Why,  we  may  ask,  does  not  twinning  occur  more 
frequently  when  the  environmental  conditions  appear  to 
be  such  as  to  favor  it  ?  It  seems  to  me  highly  probable 
that  the  same  reason  applies  here  as  in  the  birds:  that 
the  eggs  have  passed  the  critical  period  before  the}'  are 
laid.  There  is  available  some  direct  evidence  that  the 
chelonian  embryo  is  a  little  farther  along  than  is  that  of 
the  bird  at  the  time  of  laying.  Consequently  we  might 
expect  only  minimal  twinning  to  occur,  viz.,  that  in 
which  the  already  established  axis  of  symmetry  is  affected 
so  as  to  bring  about  a  more  or  less  extensive  fission  of 
the  bilateral  primordia,  especially  those  in  the  median 
dorsal  position.  If  then  scute  and  plate  irregularities 
are  to  be  interpreted  as  the  result  of  a  minimal  phase  of 
twinning,  my  earlier  interpretation  of  these  "abnormali- 
ties" as  reversions  to  an  ancestral  condition  (Newman, 
1906)  will  have  to  be  modified,  and  I  shall  haw  to  confi 
that  at  one  time  I  was  less  cautious  than  I  now  would 
be  as  to  the  interpretation  of  anomalous  or  abnormal 
biological  materials  as  evidence  of  phylogenetic  or  ana 
tral  conditions.  Most  of  us  have  at  some  time  or  oth 
fallen  into  this  familiar  type  of  error. 


98  THE  PHYSIOLOGY  OF  TWINNING 

TWINNING  IN  OTHER  CHORDATES 

Cyclostomata. — Only  one  other  class  of  vertebrates 
remains  in  which  twinning  has  not  already  been  dealt 
with:  the  round-mouth  eels.  That  twinning  occurs  here 
as  elsewhere  among  the  vertebrates  is  evidenced  by  the 
fact  that  several  papers  have  been  written  on  twinning 
in  these  forms. 

Barfurth  (1899)  has  described  a  case  of  a  larva  of 
Petromyzon  planeri  with  two  tails.  Bataillon  has  written 
a  note  on  spontaneous  blastotomy  and  conjoined-twin 
larvae  in  the  lamprey. 

Elasmobranchii. — Dohrn  (1902)  has  described  an 
interesting  double  Torpedo  embryo  which  is  of  especial 
interest  because  it  is  so  clearly  a  product  of  partial  fission. 
There  are  two  complete  notochords  but  only  one  median 
row  of  mesoblastic  somites,  belonging  equally  to  the 
two  half-embryos.  The  outer  sides  of  the  two  half- 
embryos  are  quite  complete  and  exactly  equivalent 
mirror-images  of  each  other.  Kaestner  (1898)  reports 
the  finding  of  two  eggs  of  the  selachian  Pristiurus  which 
had  two  blastoderms  on  a  single  yolk.  In  one  of  the 
eggs  the  two  blastoderms  were  of  equal  size  and  about 
one-fourth  of  an  inch  apart;  in  the  other  egg  the  two 
blastomeres  were  of  very  different  size  and  in  contact 
as  though  one  had  been  split  off  from  the  other. 

Amphioxus. — While  Amphioxus  is  not  a  true  verte- 
brate it  is  believed  to  be  the  most  closely  allied  of  the 
chordates  to  the  vertebrates.  It  is  therefore  of  interest 
to  record  briefly  in  this  place  the  well-known  work  of 
E.  B.  Wilson  on  artificial  production  of  twins  in  Amphi- 
oxus. By  shaking  the  eggs  while  in  the  two-cell  stage 
the  blastomeres  are  either  entirely  separated  so  as  to 


AMPHIBIA,  REPTILES,  OTHER  CHORDA  I  I  99 

form  completely  independent  dwarf  larvae,  or  else  they 
are  only  slightly  separated  so  that  they  cease  to  act  in 
unison  and  become  physiologically  isolated  so  as  to  form 
double  gastrulae  and  double-monster  larvae.  The  condi- 
tions are  quite  like  those  described  for  the  starfish, 
Patiria. 


CHAPTER  VIII 

THE  CAUSES  OF  TWINNING  IN  THE 
ARMADILLOS 

In  1909  we  (Newman  and  Patterson)  first  discovered 
and  studied  "  specific  poly embryony  "  in  the  nine-banded 
armadillo  of  Texas  (Dasypus  novemcinctus  texanus).  It 
was  found  that  this  species  habitually  gives  birth  to  a 
litter  of  four  offspring,  that  all  members  of  any  given 
litter  are  of  the  same  sex,  and  that  the  members  of 
any  one  litter  are  usually  strikingly  alike.  In  19 10  we 
published  a  more  detailed  study  of  the  development 
of  this  species  in  which  all  stages  from  the  primitive- 
streak  stage  to  birth  were  studied.  In  191 1  we  pub- 
lished a  statistical  study  of  variation  and  heredity  in 
armadillo  quadruplets  and  showed  that  the  members  of 
a  litter  are  as  closely  similar  to  one  another  as  are  the 
right  and  left  sides  of  single  individuals;  they  have  a 
coefficient  of  correlation  of  .9+  as  compared  with  that 
of  ordinary  siblings,  which  is  about  .5.  It  was  also 
clearly  shown  that  the  quadruplets  were  arranged  in 
two  pairs  and  that  the  two  individuals  of  a  pair  were 
more  nearly  identical  than  are  individuals  belonging 
to  opposite  pairs.  In  191 2  I  studied  the  oogenesis 
and  ovulation  of  the  armadillo  and  showed  that  only 
one  egg  is  given  off  at  a  breeding,  for  only  one  corpus 
luteum  was  formed  in  the  ovary.  The  egg  was  an 
entirely  typical  mammalian  egg.  In  19 13  I  formu- 
lated the  first  theory  ever  published  as  to  the  causes 

100 


CAUSES  OF  TWINNING  IN  ARMADILL*  I  101 

of  this  phenomenon.  In  a  general  paper  on  the  natural 
history  of  the  nine-banded  armadillo  the  view  v. 
expressed  that  the  process  of  twinning  was  due  to  a 
lowering  of  the  rate  of  metabolism  of  the  early  embryonic 
vesicle,  resulting  in  the  "  physiological  isolation  of  parts 
at  certain  distances  from  the  dominant  (apical)  region. 
When  such  isolation  occurs  new  centers  of  control  ari 
which  produce  buds  capable  of  establishing  whole  new 
systems  like  the  original."  At  that  time  no  facts  were 
available  which  seemed  to  account  for  the  lowering 
of  the  rate  of  development  of  the  embryonic  vesicle. 
Certain  peculiar  bodies,  that  were  identified  by  a  well- 
known  protozoologist  as  protozoan  parasites,  were  foun<  1 
abundantly  in  ovarian  oocytes,  and  the  suggestion 
was  made  that  these  bodies  were  the  probable  cause  of 
the  developmental  slow-down  that  initiated  twinning. 
Since,  however,  the  protozoologist  in  question  subse- 
quently withdrew  his  original  diagnosis  of  the  suppose  1 
intracellular  parasite,  this  suggested  cause  of  the  lowered 
rate  of  metabolism  had  to  be  abandoned.  Up  to  this 
time  it  was  known  that  each  set  of  quadruplets  was  the 
product  of  a  single  egg,  but  the  exact  time  and  mode 
of  twinning  was  not  definitely  known. 

We  are  indebted  to  Patterson  (1913)  for  giving  us  a 
detailed  account  of  the  twinning  process.  He  diso  >vere<  1 
in  considerable  numbers  blastocysts  in  pre-twinning 
stages  and  also  found  many  stages  of  twinning.  First 
the  originally  single  ectodermic  vesicle  elongates  in  the 
bilateral  axis  of  the  uterus  and  twin  thickenings  of  the 
apical  ectoderm  are  formed.  This  is  a  true  twin  staj 
Then  each  of  the  twins  divides  bilaterally  into  tv 
embryos,  making  two  pairs  of  twins,  or  a  set  of  quadra- 


102  THE  PHYSIOLOGY  OF  TWINNING 

plets.  Once  these  four  twin  primordia  are  established, 
each  develops  its  own  amnion,  allantois,  and  placenta; 
and  they  remain  essentially  isolated,  though  surrounded 
by  a  common  chorion,  till  birth. 

In  his  search  for  still  earlier  embryonic  stages  (the 
late  and  early  cleavage  stages)  Patterson  made  a  very 
important  observation,  the  significance  of  which  he 
failed  entirely  to  appreciate.  He  began  collecting 
earlier  and  earlier  in  the  season  for  several  successive 
years  and  found  no  earlier  stages,  but  did  find  abundant 
instances  of  single  untwinned  vesicles  lying  free  in  the 
uterus.  A  cytological  study  of  these  vesicles  showed 
that  they  were  not  developing,  since  no  mitotic  figures 
were  to  be  found  in  any  of  the  tissues.  This  "  period  of 
quiescence"  lasted  at  least  three  weeks,  and  probably 
longer. 

Here  then  was  unequivocal  support  of  my  original 
theory  that  twinning  was  due  to  a  developmental  slow- 
down, and  I  immediately  realized  the  importance  of 
this,  but  it  was  not  until  191 7  that  a  further  elabora- 
tion of  my  theory  of  twinning  was  made  public.  In  the 
volume  on  The  Biology  of  Twins  the  significance  of  the 
" period  of  quiescence"  described  by  Patterson  was 
discussed,  and  twinning  was  explained  as  the  direct 
result  of  this  period  of  quiescence.  The  view  then 
expressed  was  that,  as  the  result  of  a  very  marked 
retardation  in  the  rate  of  development,  the  original 
apical  region  lost  its  dominance  over  subordinate  regions 
and  that,  when  placentation  occurred  and  development 
was  resumed,  at  first  two  new  centers  of  growth  or  apical 
points  arose,  and  later  two  others  became  isolated;  so 
that,  instead  of  one  apical  end  or  head,  four  head  pri- 


CAUSES  OF  TWINNING  IN  ARMADILLOS        103 

mordia  were  physiologically  isolated  on  the  originally 
single  ectodermic  vesicle.  From  the  time  of  their  i 
lation  till  they  are  born  the  four  individuals  remain 
morphologically  and  physiologically  independent.  They 
are  merely  inclosed  in  a  common  chorion,  which  ante- 
dates the  twinning  process. 

For  a  considerable  time  then  I  have  steadfastly  held 
the  view  that  twinning  in  the  armadillo  is  caused  by 
arrested  development  resulting  in  a  more  or  less  complete 
obliteration  of  the  axiate  organization,  together  with 
loss  of  the  integrative  properties  of  the  original  head 
and  the  emancipation  of  subordinate  regions  from  the 
control  of  the  original  dominant  region.  This  plainly 
suggests  physiological  isolation.  I  have  no  reason  to 
abandon  this  general  view,  but  shall  attempt  to  give 
to  it  a  more  concrete  setting. 

CAUSES   OF   THE    " PERIOD    OF    QUIESCENCE'1 

The  progress  of  a  scientific  theory  is  one  that  pro- 
ceeds step  by  step  from  immediate  cause  to  causes  more 
and  more  remote.  The  immediate  cause  of  twinning  in 
the  armadillo  is  the  physiological  isolation  of  secondary 
growing-points  (apical  points) ;  the  cause  of  physiol<  tgical 
isolation  is  the  partial  obliteration  of  the  axiate  relations 
in  the  ectodermic  vesicle;  the  cause  of  this  deaxiation 
is  the  greatly  lowered  rate  of  development  incident  to 
the  "period  of  quiescence."  The  next  link  in  our  chain 
of  causes  is  the  one  that  will  account  for  the  "period  of 
quiescence."  In  his  voluminous  monograph  on  Develop- 
mental Rate  and  Structural  Expression,  Stockard  [921  I 
reviews  the  armadillo  situation  and  offers  "an  explana 
tion  of  polyembryony  in  the  armadillo, "  which  invoh 


104  THE  PHYSIOLOGY  OF  TWINNING 

certain  rather  intangible  morphological  conceptions,  such 
as  "discontinuous  mode  of  development,"  "develop- 
mental moments,"  "developmental  arrests,"  and  "alter- 
nation of  generations."  Stockard  accepts  my  view 
that  twinning  in  the  armadillo  and  elsewhere  is  a  result 
of  an  early  retardation  of  development  or  "arrested  de- 
velopment." He  also  adopts  the  budding  hypothesis  of 
Patterson,  especially  the  latter's  view  that  one  embryo 
of  each  pair  is  a  sort  of  accessory  bud  given  off  laterally 
from  the  primary  bud. 

Stockard  credits  me  with  having  appreciated  the 
significance  of  the  "period  of  quiescence"  in  twin 
formation,  but  claims  that  I  overlooked  what  he  himself 
considers  the  very  important  fact  that,  during  this 
period,  the  blastocyst  lies  free  in  the  uterus.  This,  in 
spite  of  the  fact  that  two  references  to  this  very  fact 
were  made  on  page  39  of  my  Biology  of  Twins.  That 
there  was  no  failure  on  my  part  to  appreciate  the  physio- 
logical significance  of  the  belated  placentation  is  evi- 
denced by  the  statement  on  page  88  to  the  effect  that  it 
is  only  after  the  physiological  isolation  of  the  quadruplet 
primordia  that  a  nutritive  connection  is  established 
between  the  embryonic  vesicles  and  the  maternal  tissues. 
It  seemed  to  me  at  that  time  almost  an  obvious  inference 
from  the  facts  that  the  "period  of  quiescence"  and  the 
failure  to  undergo  placentation  were  physiologically 
related,  but  no  categorical  statement  to  that  effect  was 
made. 

Stockard,  therefore,  is  to  be  credited  with  emphasizing 
this  point.  Undoubtedly  failure  to  undergo  placentation 
at  the  time  when  eutherian  mammals  usually  do  is  the 
immediate  cause  of  the  arrest  of  development.     Evi- 


CAUSES  OF  TWINNING  IN  ARMADILLOS        105 

dently  the  egg  goes  as  far  on  its  own  resources  as  ii  can, 
and  would,  in  any  species  of  mammal,  cease  to  develop 
if  unable  to  get  the  necessary  assistance  furnished  by 
placentation.  What  the  nature  of  early  placental  aids 
to  development  are  can  readily  be  conjectured.  They 
are  primarily  food  and  oxygen  and  a  means  of  eliminating 
wastes.  Doubtless,  in  the  armadillo,  the  deprivation 
of  all  of  these  growth  necessities  brings  growth  to  a  nearly 
complete  standstill.  Patterson  notes  that  cell  division 
ceases  but  that  the  vesicle  continues  to  expand  through 
the  excretion  of  liquid  within  the  cavity  of  the  vesicle. 
Protoplasmic  activity  must  be  going  on  all  this  time, 
but  cell  respiration  must  be  largely  anaerobic.  Probably 
there  is  an  accumulation  of  carbon  dioxide  and  other 
metabolic  wastes.  This  of  itself  would  tend  to  check 
development.  Stockard  claims  that  the  primary  cause 
of  the  developmental  arrest  is  lack  of  oxygen,  and  this 
may  well  be  partially  true.  The  real  cause,  however, 
is  failure  to  attain  at  the  proper  time  the  essential 
growth  stimulus  which  is  normally  supplied  by  placenta- 
tion. 

When  we  have  added  as  a  link  to  our  chain  of 
causes  that  of  belated  placentation,  we  still  must  account 
for  this  situation.  Why  does  not  the  egg  attain  a 
placental  connection  when  it  first  comes  in  contact  with 
the  maternal  mucosa  ?  This  is  a  problem  whose  solution 
would  get  at  the  very  foundations  of  the  causes  of  twin 
ning  and  might  lead  to  an  experimental  control  of 
twinning  in  mammals,  including  man.  When  blasto 
cysts  undergo  placentation  there  is  a  mutual  proliferation 
of  tissues,  both  embryonic  and  maternal.  Each  seems 
to  respond  to  stimuli  given  off  by  the  other.     Ii  eith< 


106  THE  PHYSIOLOGY  OF  TWINNING 

were  non-sensitive  no  placentation  would  result.  In 
this  case,  I  believe,  the  non-sensitiveness  is  not  of  the 
embryo  but  of  the  maternal  mucosa.  It  is  slow  to 
acquire  the  sensitivity  that  a  uterus  normally  possesses 
in  the  presence  of  a  blastocyst.  Recent  experimental 
work  has  tended  to  show  that  the  sequence  of  events  in 
mammalian  gestation  is  controlled  by  an  intricate 
system  of  hormones.  The  event  of  ovulation  is  quickly 
followed  by  the  formation  of  a  glandular  body,  the 
corpus  luteum,  which  appears  to  control  by  its  secretions 
further  ovulation  and  to  excite  the  uterine  mucosa  to 
co-operate  with  the  blastocyst  in  placentation.  If,  for 
any  reason,  the  functioning  period  of  the  corpus  luteum 
should  be  delayed  there  would  inevitably  follow  a  delay 
in  placentation.  In  my  opinion  this  is  a  point  of  experi- 
mental attack.  If  my  assumption  be  correct,  early 
removal  of  the  corpus  luteum  might  be  expected  to 
inhibit  placentation.  In  the  armadillo  we  probably 
have  a  case  of  sluggish  activity  of  the  corpus  luteum; 
for  it  grows  very  large  and  becomes  very  active  at  a 
later  period,  and  excellent  placentation  finally  results. 
Even  if  it  should  prove  to  be  true  that  slow  growth  and 
sluggish  activity  of  the  corpus  luteum  is  a  part  of  our 
causal  chain  we  would  still  have  to  discover  the  cause 
of  this.     The  search  for  causes  is  endless. 

The  essentials  in  the  causal  chain  of  twinning  in  the 
armadillo,  according  to  the  foregoing  theory,  are  as 
follows:  (a)  slow  development  or  sluggish  functioning 
of  the  corpus  luteum;  (b)  failure  of  the  maternal  mucosa 
to  respond  to  the  presence  of  the  blastocyst;  (c)  belated 
placentation;  (d)  cessation  of  development  for  about 
three  weeks;    (e)  partial  loss  of  polarity  or  deaxiation 


CAUSES  OF  TWINNING  IN  ARMADILLOS         107 

of  the  ectodermic  vesicle;   (/)  the  physiological  Isolation 
of  two,  then  four,  growing  regions  on  the  blastoderm; 
(g)  the  independent  development  from  the  four  growing 
points  of  four  complete  embryos. 

Almost  equally  important  for  an  understanding  of 
our  problem  is  the  very  obvious  renewal  of  developmental. 
activity  as  soon  as  placentation  is  attained.  The  embrv. 1 
is  first  aroused  only  locally,  in  the  region  of  the  "Trager.  " 
but,  with  the  renewed  vigor  acquired  through  the 
establishment  of  this  first  placental  connection,  the  wave 
of  renewed  growth  energy  sweeps  distally  through  the 
tissues  and  ultimately  strikes  the  ectodermic  vesicle. 
This  region  of  the  embryo  has  been  the  most  profoundly 
inhibited,  partly  owing  to  its  inclosed  position  and  partly 
because  the  ectoderm,  and  particularly  the  nervous 
ectoderm,  is  the  most  susceptible  region  of  the  embryo 
to  growth-depressing  agencies.  It  is  this  vesicle  in 
which  we  get  our  first  visible  evidences  of  twinning, 
though  it  is  my  belief  that  the  true  physiological  isolation 
of  the  quadruplet  primordia  considerably  antedates  their 
visible  or  morphological  isolation.  Once  this  isolation 
is  accomplished  twinning  has  occurred. 

To  summarize,  then,  there  is  a  fairly  general  agree 
ment  among  those  of  us  who  are  interested  in  the 
causes  of  twinning  that,  in  the  armadillo,  the  funda- 
mental cause  is  interrupted  or  retarded  development  at 
a  critical  period,  followed  by  an  isolation  of  four 
growing-points.  The  exact  mechanism  of  isolating  the 
four  growing-points  is  still  undetermined.  Two  theories 
prevail  at  present:  the  budding  theory  of  Patterson 
and  of  Stockard,  and  the  fission  theory  oi  which  I  am 
an  advocate. 


108  THE  PHYSIOLOGY  OF  TWINNING 

THE   FISSION   THEORY  VERSUS   THE   BUDDING  THEORY 
OF   TWINNING   IN   THE   ARMADILLO 

At  the  present  time  two  writers,  Patterson  and 
Stockard,  hold  to  the  budding  theory,  while  Assheton 
and  the  writer  feel  that  the  process  is  not  one  of  budding 
but  of  fission.  Assheton  says:  "One  cannot  have 
budding  unless  there  is  a  stock  from  which  budding 
takes  place.  There  is  nothing  in  Tatusia  (Dasypus)  one 
can  call  a  stock.  The  phenomenon  is  clearly  that  of 
fission."  To  this  Stockard  replies:  "The  use  of  the 
word  bud  or  budding  in  connection  with  double  embryo- 
formations  as  employed  by  Patterson  (19 14)  has  been 
criticized  by  Assheton,  who  suggests  fission  as  the  better 
word  for  the  process.  Such  a  discussion  seems  devoid 
of  value  and  I  employ  the  word  bud  to  mean  what  is 
indicated  above." 

It  is  evident  from  this  expression  of  Stockard's  that 
he  regards  the  distinction  between  budding  and  fission 
as  a  valueless  splitting  of  hairs.  I  feel  quite  the  opposite. 
To  me  the  interpretation  of  armadillo  twinning  as  a 
budding  process  is  extremely  misleading  and  involves  a 
total  misapprehension  of  the  significance  of  the  nature 
of  twinning.  In  order  that  we  may  be  on  solid  ground 
in  this  discussion  it  becomes  necessary  to  re-examine  in 
detail  the  actual  facts  upon  which  the  theories  of  twinning 
are  based.  We  are  indebted  to  Patterson  for  his  excellent 
description  of  the  facts.  It  is  only  his  interpretations 
that  seem  to  me  to  be  incorrect. 

The  armadillo  blastocyst,  just  before  it  shows 
morphological  evidences  of  twinning,  is  represented  in 
Figure  46.  It  is  now  connected  with  the  maternal 
mucosa  by  means  of  the  Trager  ring.     Cell  division  has 


CAUSES  OF  TWINNING  IN  ARMADILI  <  109 


been  resumed  and  it  is  high  time  that  an  embryonic  axis 
developed.  There  is,  however,  no  clearly  defined  head 
process  or  primitive  streak.     The  ectodermic   vesicle, 

Tra 

Ml 


Fig.  46. — Section  of  an  armadillo  embryo,  cut  at  right  angles  to  the 
sagittal  plane,  showing  the  first  evidences  of  the  prosper  tive  twinning 
process.  A  secondary  bilaterality  has  begun  to  be  imposed  upon  the 
embryo  by  the  bilaterality  of  the  uterus.  The  point  X  is  the  original 
apical  end  of  the  single  embryo.     The   two  sides  of  thi  lermic 

vesicle  (ec)  are  thicker  than  the  point  X  and  are  the  blastoderms  of  the 
twins.     Tra,  Trager  or  primitive  placenta;  e»,  endoderm;   tr  pi,  tropho- 
derm  plate;   ec  am,  ectodermal  layer  of  amnion;   am  C,  amniotic  ca> 
ms,  mesoderm;    ys,  yolk,  sac;    ex  c  extra-embryonic  cavity. 
Newman  after  Patterson.) 


no  THE  PHYSIOLOGY  OF  TWINNING 

which  must  be  thought  of  as  homologous  with  the 
medullary  plate  rolled  up  into  a  hollow  ball,  has  thinned 
out  in  the  roof  to  form  the  ectodermal  layer  of  the 
amnion.  The  center  of  the  floor  is  the  region  where 
we  would  expect  to  see  the  first  sign  of  a  differentiation 
of  the  apical  point  of  the  new  axis;  but  this  part  is  even 
a  little  thinner  than  are  the  lateral  walls,  directly  to 
the  right  and  to  the  left  of  the  original  apical  region. 
The  only  evidence  of  a  definite  bilaterality  in  the  ecto- 
dermic  vesicle  is  seen  in  the  fact  that  it  is  broader  in  one 
plane  than  in  any  other.  The  figure  is  drawn  in  this 
plane  and  shows  in  the  mesoderm  further  evidences 
either  that  bilaterality  has  persisted  in  the  vesicle  or 
else  that  it  has  been  established  de  novo  as  a  result  of  the 
position  of  the  vesicle  with  respect  to  the  axis  of  the 
uterus;  for  the  bilateral  axis  of  the  embryo  coincides 
with  that  of  the  uterus  even  at  this  early  period.  The 
mesoderm  begins  to  proliferate  from  two  points  where  the 
ectoderm  and  endoderm  part  company.  For  some  time 
the  mesoderm  consists  of  a  considerable  number  of 
isolated  thin-walled  vesicles,  but  there  is  always  a  period 
when  these  small  vesicles  break  together  into  two  large 
vesicles,  separated  by  a  median  mesentery  that  coincides 
with  the  principal  axis  of  the  untwinned  embryo.  Con- 
cerning this  point  it  is  important  to  note  Patterson's 
comment: 

The  earliest  observed  evidence  which  could  be  interpreted 
as  representing  the  beginning  of  multiple  embryos  comes  in  the 
formation  of  the  mesothelium — not  in  the  manner  in  which  the 
elements  of  this  layer  arise,  for  localized  centers  of  proliferation 
were  not  found,  but  in  the  early  formation  of  two  large  mesodermal 
vesicles  through  the  fusion  of  smaller  ones.  The  development  of 
two  mesodermal  vesicles  would  not  in  itself  be  so  significant,  as  it 


CAUSES  OF  TWINNING  IN  ARMADILLOS        1 1  i 

might  be  merely  an  expression  of  a  bilateral  arrangement  of  nu 
derm  similar  to  that  of  many  other  vertebrate  embryos  [italics  mine], 
were  it  not  for  the  fact  that  they  hold  a  position  corresponding 
exactly  to  the  two  primary  ectodermal  buds;  that  is,  they  lie  on 
the  sides  of  the  vesicle  which  are  directed  toward  the  openings  of 
the  Fallopian  tubes. 

This  important  statement  seems  to  me  to  settle  at 
least  two  questions  that  have  puzzled  us  for  some  time. 
The  first  is  that  we  have  revealed  to  us  the  origin  of 
the  exact  correspondence  existing  between  the  two  pairs 
of  twins  and  the  two  halves  of  the  uterus.  It  will  be 
recalled  that  there  are  always  two  fetuses  attached  to  a 
right-hand  placental  disk  and  two  more  to  the  left-hand 
disk.  These  are  natural  twin  pairs  and  show  many 
evidences  of  an  extremely  close  relationship.  It  is  al- 
most impossible  to  conceive  of  the  vesicle  as  a  bilateral 
organism  coming  to  lie  in  such  a  fashion  as  to  have  it- 
plane  of  symmetry  coincide  with  that  of  the  uterus. 
The  alternative  view,  and  one  that  agrees  well  with  the 
facts,  is  that,  either  after  its  long  period  of  quiescence 
the  vesicle  has  lost  any  bilaterality  that  it  may  have 
possessed,  or  that  up  to  that  time  it  had  never  developed 
an  axis  of  symmetry.  Only  the  axis  of  polarity  had 
been  established  and  this  had  been  nearly  obliterated. 
The  crosslike  placental  area  of  the  uterus  (see  The 
Biology  of  Twins,  p.  31)  is  very  precise  in  its  topographic 
outlines  and  it  seems  clear  that  the  vesicle  soon  comes 
to  be  influenced  by  its  location  in  such  a  way  that  a  new 
symmetry  system  arises  in  response  t<>  the  symmetrical 
conditions  of  the  uterine  environment.  That  tin-  envi 
ronment  does,  in  certain  cases  at  least,  determine  t bi- 
axial and  symmetrical  relations  of  developing  organisms 
has  been  repeatedly  demonstrated  by  various  an  tlmrs.  and 


ii2  THE  PHYSIOLOGY  OF  TWINNING 

that  this  is  only  another  instance  of  a  very  general  situ- 
ation seems  evident.  Much  of  the  difficulty  formerly 
associated  with  the  fact  that  the  symmetrical  relations 
of  armadillo  quadruplets  coincide  with  those  of  the  mother 
thus  disappears  when  we  view  the  fetal  symmetry  as  de- 
termined de  novo  by  that  of  the  uterus.  The  second  point 
of  importance  that  is  brought  out  by  Patterson's  descrip- 
tion inheres  in  the  italicized  words,  for  I  believe  that  the 
bilateral  arrangement  prior  to  twinning  which  he  points 
out  is  not  at  all  the  bilaterality  of  the  untwinned  embryo, 
but  merely  the  result  of  a  physiological  isolation  of  two 
halves  of  the  vesicle.  It  is  the  equivalent  of  what 
happens  in  a  starfish  blastula  when  two  invagination 
areas  become  physiologically  isolated  so  as  to  lie  in 
equivalent  positions  to  each  other,  so  that  each  faces 
the  other  like  a  pair  of  mirror-images.  The  separation 
of  the  two  gastrulation  areas  is  obviously  a  sort  of 
migration  of  cells  toward  the  right  and  left  of  the  uterus, 
leaving  a  thinned-out  region  in  the  middle  line.  This  is 
more  like  a  fission  process  than  a  budding  process,  for 
of  the  two  embryonic  areas  it  would  be  impossible  to  say 
which  is  the  original  individual  and  which  is  the  bud. 
The  concept  of  budding  implies  that  the  original  apical 
point  retains  its  identity  and  that  the  bud  is  a  secondary, 
more  or  less  lateral,  new  growing-point  that  has  escaped 
from  the  dominance  of  the  original  or  primary  growing- 
point  and  has  asserted  its  own  independence.  This  is 
evidently  Patterson's  idea  of  budding,  at  least  in  so  far 
as  the  formation  of  secondary  buds  is  concerned,  as  will 
be  made  clear  from  the  following  quotation : 

The  primary  buds  do  not  develop  for  some  time  after  the 
completion  of  the  ectodermal  vesicle,  although  their  appearance 


CAUSES  OF  TWINNING  IN  ARJMADILLC  113 

is  anticipated  soon  after  this  period  by  certain  easily  detectable 
changes  in  the  walls  of  the  vesicle.  It  will  be  recalled  thai  immedi- 
ately after  the  ectodermal  sphere  has  become  transformed  into 
a  vesicle,  that  portion  of  the  wall  of  the  vesicle  which  is  turned 
toward  the  free  pole  of  the  blastocyst  is  of  a  relatively  uniform 
thickness.  Very  shortly  thereafter  one  can  detect  a  tendency  in 
this  region  of  the  wall  to  become  less  thick.  The  thinning  out 
may  be  due  in  part  to  an  increase  in  size  of  the  vesicle  by  the 
accumulation  of  fluid  within  its  cavity,  but  undoubtedly  in  tin- 
main  it  is  brought  about  through  the  shifting  of  cells  from  here  to 
the  lateral  portions  of  the  wall,  for  these  show  an  increase  in  thick- 
ness. 

The  shifting  of  cells  from  the  pole  of  the  vesicle  results  in 
the  formation  of  a  thickened  zone  adjoining  the  thin  or  endothelial- 
like  portion  of  the  ectodermal  vesicle.  The  zone  is  not  uniformly 
thick,  but  is  thickest  at  the  two  regions  corresponding  respectively 
to  the  right  and  left  sides  of  the  vesicle.  One  can  therefore 
correctly  speak  of  these  thickened  areas  as  lateral  plates. 

The  primary  buds  arise  from  these  lateral  plates,  and  appear 
as  two  broad,  blunt  processes  protruding  from  the  sides  of  the 
ectodermal  vesicle.  Each  bud  involves  the  greater  portion  of  the 
side  of  the  vesicle,  covering  an  arc  of  approximately  80  degrees 
on  the  circumference  [Fig.  47,  p.  114]. 

What  better  description  of  &  fission  process  could  one 
ask  for  than  this?  The  pre- twinning  stage  of  the 
vesicle,  which  it  must  be  remembered  has  undergone 
germ-layer  inversion  so  that  the  cells  originally  apical 
in  position  are  now  occupying  the  distal  pole  <>f  the 
ectodermic  vesicle,  is  characterized  by  the  fact  that  the 
polar  area  is  thickest.  This  was  the  prospective  locus 
of  the  head  of  the  untwinned  embryo.  Then  this  apical 
area  becomes  relatively  thinner  down  the  middle  and 
two  bilaterally  arranged,  thickened  areas  arise  and  are 
distinctly  separated  by  the  median,  thinned-out  area. 
Is  this  budding  or  fission?     The  so-called  primary  buds 


ii4 


THE  PHYSIOLOGY  OF  TWINNING 


are  merely  the  bulges  due  to  the  presence  of  the  two 
thickened  areas,  the  medullary  plates  of  the  now  twin 
embryos.  This  is  the  true  twin  stage  in  armadillo 
development.  Before  any  further  differentiation  occurs, 
however,  these  twin  embryonic  areas  once  more  undergo 
bilateral  fission,  rather  less  complete  than  the  first 
fission,  for  the  median  area  does,  not,  for  a  time  at  least, 


Tra 

Mm?* 


-en 


Fig.  47. — An  armadillo  embryo  in  the  true  twin  stage.  The  thick- 
ened plates  of  ectoderm  below  the  figures  II  and  IV  are  the  embryonic 
primordia  of  the  twin  embryos  and  are  as  yet  undivided  to  form  the 
quadruplet  condition.  Lettering  same  as  in  Fig.  46.  (From  Newman 
after  Patterson.) 


CAUSES  OF  TWINNING  IN  ARMADILU  >S         I  i 


thin  out  so  markedly  as  when  the  first  twinning  proce 
occurred.  The  twinned  embryonic  areas,  one  od  each 
side  of  the  vesicle,  do  not  completely  separate  but 
remain  united  by  a  relatively  thick  band  of  ectoderm 
and  it  is  the  gradual  severing  of  this  connection,  beginning 
at  the  posterior  end,  that  Patterson  has  mistakenly 
interpreted  as  a  secondary  budding  process.  His  own 
account  shows  this: 

The  formation  of  the  secondary  buds  immediately  follows 

the  establishment  of  the  primary  diverticula Each  primary 

bud  gives  rise  to  two  second- 
ary buds,  and  consequently 
there  are  four  secondary 
diverticula.  Each  secondary 
bud  carries  the  rudiment  or 
primordium  of  an  embryo. 
The  first  step  leading  to  the 
development  of  the  secondary 
diverticula  consists  in  the 
formation  of  two  thickenings 
in  the  wall  of  each  primary 
bud.  One  of  these  areas  lies 
at  the  tip  of  the  bud,  while 
the  other  appears  slightly  to 
the  left  (as  viewed  from 
above)  of  the  tip.  The  sec- 
ondary buds  then  arise  from 
these  areas  as  blind  diver- 
ticula, which  extend  down 
along  the  inner  surface  of  the 
yolk-sac  entoderm. 


Fig.  48. — Outline  polar  view  of  an 
armadillo  quadruplet  egg  after  the 
completion  of  the  process  of  twin- 
ning. The  four  embryonic  anas  are 
entirely  separate;  each  has  established 
its  own  axis  and  is  about  to  migrate 
backward  toward  the  placental  region. 


The  head  ends  of  the  four  embryos 
Let  us  look  for  a  mo-     point  toward  the  center  of  the  vesk  le. 

ment  at  Patterson's  fig-     The  Postt-'rior  enda  are  beginning  to 

.„  .        ..       r  push  outward  and  give  rise  to  the 

ure  illustrating  the  four    aiUcd  „buds„  o(  Pattera01L    (After 

"buds"    (Fig.   48).      The      Patterson.) 


n6  THE  PHYSIOLOGY  OF  TWINNING 

first  thing  of  importance  that  I  note  is  what  Patterson  has 
apparently  forgotten :  that  the  apical  or  head  ends  of  the 
four  embryos  are  all  directed  inward  toward  the  center 
of  the  vesicle.  The  heads  are  already  widely  separated 
as  are  also  the  embryonic  areas.  The  "buds"  are 
merely  outpushings  at  the  posterior  ends  of  the  embryos 
involving  largely  extra-embryonic  (amniotic)  ectoderm. 
It  should  be  entirely  obvious  from  Patterson's  own 
account  that  the  essential  acts  of  twinning  are  entirely 
finished  before  this  " budding"  process  begins.  What 
then  are  these  so-called  "buds"  ? 

This  question  may  be  readily  answered  after  studying 
the  typical  embryonic  history  of  non- twinning  armadillos, 
for  it  is  here  that  the  clue  to  their  interpretation  exists. 
According  to  Fernandez  (19 14),  who  has  studied  the 
development  of  the  non-twinning  species  Euphractus 
villosus,  the  medullary  plate  of  the  single  embryo  arises 
from  the  distal  thickening  of  the  ectodermic  vesicle, 
just  as  it  begins  to  do  in  our  twinning  species.  Instead, 
however,  of  remaining  at  the  distal  pole,  farthest  from 
the  placenta,  the  embryo  grows  backward  out  of  the 
amnion,  which  remains  attached  to  the  distal  endoderm. 
It  leaves  the  amnion  by  means  of  a  process,  the  "bud'; 
of  Patterson.  Not  only  does  the  posterior  end  grow 
backward  but  the  head  as  well  moves  along  the  inner 
wall  of  the  ectoderm  carrying  attached  to  it  an  amniotic 
connecting  canal  that  remains  as  a  hollow  string  connect- 
ing the  amnion  of  the  embryo  to  the  rudiment  of  the 
original  amnion  at  the  opposite  pole.  It  is  evident, 
therefore,  that  Patterson's  "secondary  buds"  are  merely 
the  beginnings  of  amniotic  outpouchings  which  are 
destined  to  act  as  migration  canals  through  which  the 


CAUSES  OF  TWINNING  IN  ARMADILD  117 

individual  embryos  may  pass  in  order  to  reach  i: 
opposite  end  of  the  vesicle,  where  lies  thr  Tra^'r.  I  ich 
embryo  in  our  twinning  species,  in  thus  migrating  to  the 
placental  pole  of  the  vesicle,  behaves  just  as  if  it  were 
the  only  embryo  in  the  vesicle.  It  seems  clear  then 
that,  unless  we  are  prepared  to  call  the  outpouching  of 
the  non-twinned  embryo  of  Euphractus  a  "bud,,:  the 
term  "bud"  for  the  homologous  structure  of  Dasypus 
is  entirely  a  misnomer.  It  seems  strange  also  to  think 
of  embryonic  primordia  budding  at  the  tail  end,  as  they 
would  be  doing  if  this  is  budding;  for  in  typical  cases 
of  budding  it  is  implied  that  the  budding  region  is  a  new- 
apical  region  or  head  region.  It  is  quite  certain  that  in 
all  of  Stockard's  work  he  considers  his  "buds"'  as  new 
head  regions. 

When  the  embryos  first  begin  their  backward  migra- 
tion toward  the  Trager,  the  paired  embryos  of  the  right 
side  remain  broadly  attached  by  a  band  of  ectoderm. 
The  same  is  true  of  the  pair  on  the  left  side.  They 
therefore  migrate  together  for  a  short  distance  and  are 
in  the  same  amniotic  canal.  Soon,  however,  the  connect- 
ing band  thins  out  and  breaks  apart  in  forklike  fashion, 
and  the  two  embryos  proceed  to  grow  and  migrate  down 
two  distinct  canals,  tail  first,  as  though  switched  back- 
ward onto  diverging  tracks.  There  is  left,  therefor 
for  a  little  distance  from  the  common  amniotic  vesicle,  a 
common  canal  which  soon  splits  into  the  two  individual 
connecting  canals.  One  can  always  identify  the  twin 
products  of  one  side  of  the  vesicle  by  the  fact  that  their 
individual  connecting  canals  thus  unite  before  entering 
the  common  amnion.  It  is  this  forking  apart  of  the 
posterior  ends  of  the  embryos  that  gives  the  appearance 


n8  THE  PHYSIOLOGY  OF  TWINNING 

of  budding.  Patterson  has  noted  that  buds  II  and 
IV  seem  to  occupy  the  original  site  of  the  "primary 
buds,"  that  is,  respectively  toward  the  right  and  toward 
the  left,  while  buds  I  and  III  appear  to  come  off  slightly 
behind  and  to  the  left  of,  respectively,  buds  II  and  IV. 
He  is  therefore  inclined  to  consider  buds  II  and  IV  as 
continuations  of  the  original  twin  primordia,  the 
"primary  buds, ':  and  that  I  and  III  are  lateral  or 
accessory  buds.  In  support  of  this  view  it  is  not  infre- 
quently noted  that  I  and  III  are  usually  not  quite  so 
advanced  as  are  II  and  IV.  The  lateral  budding  notion 
can  hardly  continue  to  be  valid  in  view  of  what  has 
already  been  stated,  but  it  is  true  that  the  position  of 
one  of  each  pair  remains  lateral  and  the  other  lies  to  its 
left.  There  is  evidently  some  very  accurately  balanced 
behavior  here,  whatever  its  significance.  The  result 
is  that  each  embryo  comes  to  occupy  its  full  quadrant 
of  the  vesicle.  What  appears  to  happen  is  that  one  of 
the  migrating  embryos  keeps  the  original  direction  while 
the  other  is  shunted  off  at  a  considerable  angle.  Why 
in  both  cases  the  shunted-off  individual  goes  to  the 
left  is  a  question  that  at  present  I  am  entirely  unable  to 
explain.  A  careful  re-examination  of  my  own  mate- 
rial and  of  some  of  Patterson's  figures  leaves  it  an 
open  question  in  my  mind  whether  the  right  embryo 
of  each  pair  always  retains  the  original  direction  of 
growth  and  the  left  is  caused  to  diverge.  There  are 
certainly  some  cases  in  my  possession  where  the  point 
of  attachment  of  the  placenta  is  more  nearly  lateral 
in  the  left-hand  embryo  than  in  the  right  and  there  are 
undoubtedly  many  instances  in  which  the  left-hand 
embryo  of  a  pair  is  more  advanced  in  development  than 


CAUSES  OF  TWINNING  IN  ARMADILLOS        119 

is  the  right.     It  would  be  interesting  to  know  just  how- 
general    is    the    condition    which    Patterson    describes. 

The  preceding  interpretation  of  twinning  disposes,  I 
believe,  of  the  lateral  budding  idea  of  Patterson.  ( lertain 
evidences  favoring  the  fission  theory  have  been  presented 
in  passing,  but  the  main  evidences  that  twinning  in  the 
armadillo  is  a  case  of  longitudinal  or  bilateral  fission 
inhere  in  the  facts  brought  out  fully  by  the  writer  several 
years  ago  in  connection  with  closeness  of  resemblance 
and  with  symmetry  reversal.  It  was  shown  quite 
conclusively  that  the  resemblances  between  these  twins 
are  as  close  as  are  the  right  and  left  sides  of  a  single 
individual,  very  closely  approximating  complete  identity. 
It  was  shown,  in  addition,  that  asymmetrical  peculiarities 
occurring  on  one  side  of  one  twin  were  very  frequently 
found  on  the  opposite  side  of  the  other  twin.  If  there 
is  any  validity  at  all  to  this  idea  of  symmetry  reversal 
it  is  obvious  that  it  must  have  significance  as  the  residue 
of  a  former  bilateral  symmetry  of  the  undivided  embry- 
onic axis,  and  it  is  equally  obvious  that  the  only 
kind  of  division  that  would  preserve  such  an  original 
symmetry  and  show  it  in  mirror-imaging  is  the  method 
of  bilateral  fission.  Lateral  budding  could  never  have 
any  such  results. 

Stockard's  theory  of  twinning  in  the  armadillo  is 
based  on  Patterson's  budding  theory,  and  is  therefore 
open  to  the  same  objections.  A  few  ({notations  from  his 
principal  paper  will  reveal  this  point  of  view : 

The  douhle  primitive  streaks  in  the  hen's  egg  and  other 
forms  all  lend  themselves  to  strengthen  the  interpretation  that 
double  embryo-formation  first  asserts  itself  by  a  double  gastrula- 
tion  or  blastopore  formation,  which  is  initially  a  pro  A  doub 


120  THE  PHYSIOLOGY  OF  TWINNING 

instead  of  single  bud  formation.  Patterson's  description  of  the 
origin  of  the  quadruplet  buds  in  the  Texas  armadillo  furnishes 
the  most  striking  case  in  the  study  of  these  conditions.  And  we 
may  conclude  that  the  budding  or  accessory  embryo-formation  in 
the  egg  of  the  armadillo  is  exactly  the  same  developmental  process 
as  that  which  gives  rise  to  twins  and  double  individuals  in  other 
vertebrate  eggs. 

In  another  place  he  says:  " There  is  reason  to  believe 
that,  aside  from  the  external  factors  discussed,  the 
armadillo  egg  is  highly  disposed  toward  the  formation  of 
accessory  embryonic  buds.1'  Again,  in  attempting  to 
explain  why  the  deer,  although  it  has  a  "period  of 
quiescence,"  fails  to  produce  twins,  he  says:  "The  egg 
of  the  deer  may  possess  only  a  very  slight  tendency 
toward  accessory  embryo  formations." 

Exactly  what  does  Stockard  have  in  mind  when  he 
uses  the  term  accessory  bud?  It  is  clear  that  he  uses 
the  term  advisedly  and  means  to  imply  just  this:  that 
the  original  embryo  retains  its  identity,  but  that, 
through  its  loss  of  dominance  over  the  rest  of  the  blasto- 
derm, accessory  or  secondary  buds  arise  which  give 
rise  to  additional  embryos.  Evidently  this  is  part  and 
parcel  of  the  theory  of  budding,  for  a  bud  is  essentially 
an  offshoot  of  a  previously  existing  individual  or  of  a 
common  stock.  If,  therefore,  the  phenomenon  of  twin- 
ning in  the  armadillo  turns  out  not  to  be  budding  at 
all,  but  fission,  the  whole  budding  theory,  together  with 
the  causal  theories  based  upon  it,  falls  to  the  ground. 


CHAPTER  IX 
THE  MODES  AND  CAUSES  OF  HUMAN  TWINNING 

INTRODUCTION 

We  recognize  two  types  of  twinning  in  man  one- 
egg  twinning  and  two-egg  twinning.  In  our  present 
discussion  we  shall  limit  ourselves  to  one-egg  twinning; 
for  two-egg  twinning  is,  strictly  speaking,  not  true 
twinning  at  all. 

It  has  now  come  to  be  very  generally  agreed  that 
separate  one-egg  twins  (duplicate  or  identical  twins) 
belong  to  the  same  series  and  result  from  the  same  causes 
as  conjoined  twins  or  double  monsters.  If  this  con- 
clusion is  valid,  acceptable  theories  of  the  modes  and 
causes  of  twinning  must  be  in  conformity  with  conditions 
in  both  types  of  twins.  It  is  my  belief  that  the  clue  to 
the  mode  of  human  twinning  must  come  from  a  study 
of  the  various  incomplete  stages  of  twinning  exhibited 
by  conjoined  twins. 

THE   MODE   OF   ORIGIN   OF   CONJOINED   TWINS 

For  a  long  time  it  has  been  tacitly  assumed,  and 
rightly  so,  that  conjoined  twins  are  the  products  of  some 
kind  of  division  of  a  single  egg.  The  grounds  for  this 
assumption  are:  (a)  they  are  always  of  the  same  sex; 
(jb)  they  very  frequently  show  situs  inversus  viscerum; 
(c)  they  are  usually  cosmobia,  i.e.,  they  arc  joined  in 
symmetrical  positions  with  regard  to  one  another,  and 
homologous  parts  of  the  two  systems  are  always  united. 

121 


122  THE  PHYSIOLOGY  OF  TWINNING 

Conjoined  twins  are  united  in  a  great  variety  of 
ways.  By  far  the  commonest  condition  is  that  in  which 
the  anterior  parts  are  separate  and  the  posterior  parts  are 
fused.  There  are,  however,  rare  cases  of  twins,  called 
Janus  monsters  in  which  the  heads  are  less  completely 
separate  than  are  some  of  the  more  posterior  parts  of  the 
body.  It  is  not  fair  to  say  that  these  individuals  have 
a  single  head  and  two  complete  bodies,  for  even  the  head 
is  double  in  that  there  are  two  faces. 

When  twins  are  united  they  are  usually  connected 
by  ventral  regions  of  the  body  though  they  may  later 
come  to  lie  in  such  a  way  that  they  fail  to  show  this 
condition  clearly.  There  is  no  certain  case  in  which  the 
dorsal  parts  of  the  twins  remain  single  while  the  ventral 
parts  are  double.  There  is  nearly  always  dorsal  dupli- 
city and  ventral  unity.  Even  in  the  cases  of  so-called 
pygopagi,  in  which  the  twins  seem  to  be  joined  back  to 
back,  the  conjoined  organs  are  not  the  vertebral  column 
nor  the  central  nervous  system,  but  are  nearly  always 
certain  ventral  structures  such  as  the  intestine  or  the 
urethra.  Rare  cases  occur  in  which  otherwise  com- 
plete twins  are  lightly  united  in  the  head  region,  as  for 
example  in  the  region  of  the  forehead  or  the  top  of  the 
head. 

If,  instead  of  attempting  to  classify  and  to  interpret 
these  double  monsters  on  the  basis  of  the  degree  of  more 
or  less  mechanical  union  which  they  exhibit,  we  give 
attention  to  the  degree  of  duplicity  of  the  various  parts 
of  the  body  and  of  its  organs,  the  problem  becomes  much 
simpler. 

Are  conjoined  twins  to  be  viewed  as  incompletely 
fused  separate  individuals  or  as  incompletely  divided 


MODES  AND  CAUSES  OF  HUMAN  TWINNING     i 

single  individuals?     The  decision  between  these  alter- 
natives is  crucial  for  our  theory. 

Granted  that  they  are  derived  from  a  single  egg,  do 
the  two  parts  of  such  twins  come  from  two  independently 
arising  embryonic  axes  which  subsequently  come  to  fuse 
together  in  certain  regions  and  remain  separate  in  others; 
or  do  they  arise  as  the  result  of  a  more  or  less  complete 
separation  of  the  bilateral  halves  of  a  single  embryonic 
axis?  The  first  alternative,  which  involves  the  idea  of 
fusion  of  separate  embryos,  meets  with  an  almost  insuper- 
able obstacle  on  account  of  "the  complete  bilateral 
symmetry  of  the  two  components  in  true  double  monsters 
(diplopagi),  since  there  is  no  force  to  oversee  and  adjust 
the  two  components  in  the  exact  relationship  necessary 
for  this  result"  (Wilder,  1904).  There  may  be,  how- 
ever, a  minor  degree  of  purely  external  or  mechanical 
fusion  due  to  previously  separated  parts  remaining  too 
closely  approximated.  Such  embryos  may  be  pushed 
or  crowded  together,  for  they  lie  within  a  single  amnion 
and  have  no  means  of  avoiding  contacts.  For  enlighten- 
ment as  to  the  mode  of  origin  of  human  double  monsters 
we  may  profitably  turn  to  the  much  better  understood 
conditions  in  the  birds;  for  we  have  reason  to  believe 
that  mammalian  conditions  are  quite  similar  in  most 
respects  to  those  of  reptiles  and  bird-.  There  were,  it 
will  be  recalled,  two  types  of  avian  double  monsters  in 
which  there  was  more  or  less  unity  of  the  head  regions 
and  duplicity  of  the  trunk  region.  The  first  of  th< 
types  was  explained  on  the  basis  of  the  existence  of  two 
more  or  less  separate  bilateral  growing  regions,  the  head 
process  and  the  primitive  streak.  Either  region  may 
undergo  complete  or  partial   twinning  without    th< 


124  THE  PHYSIOLOGY  OF  TWINNING 

operation  of  the  other.  The  second  type  of  double 
monster  was  due  to  the  mechanical  head-on  collision  of 
two  embryonic  axes  as  in  Figure  37.  So  far  as  I  am 
aware  there  are  no  cases  of  human  double  monstrosity 
that  are  to  be  explained  as  due  to  crude  mechanical 
fusions  of  the  latter  sort;  hence  it  would  appear  that 
all  cosmobia  are  to  be  explained  as  products  of  fission  or 
physiological  isolation  of  the  bilateral  primordia.  Janus 
monsters,  with  their  two  faces  and  with  brains  partially 
double  and  partially  fused,  but  with  bodies  more  com- 
pletely double  (see  The  Biology  of  Twins,  Fig.  2,6),  are  to 
be  viewed  as  instances  of  the  fission  of  the  head  process 
and  of  the  primitive  streak  in  which  the  latter  was  more 
complete  than  the  former.  The  cyclopian  monster  shown 
in  a  of  the  same  figure  is  probably  another  illustration  of 
the  independence  of  the  two  twinning  regions.  The  bodies 
are  completely  isolated  except  for  external  fusions,  while 
the  head  is  not  double  at  all,  but  just  the  opposite  in 
that  even  normally  paired  structures  such  as  the  eyes 
are  single.  The  same  factor  that  produced  cyclopia  in 
the  head  region  has  evidently  produced  twinning  in  the 
secondary  growing  region,  the  primitive  streak.  This  is 
rather  a  striking  confirmation  of  the  theory  that  twinning 
and  single  monstrosities  are  due  to  the  same  cause — 
retarded  development.  A  very  large  percentage  of 
human  double  monsters  are  classed  as  anadidymi,  in 
which  the  anterior  parts  are  more  double  than  the 
posterior  parts.  This  was  even  more  strikingly  the  case 
for  fish  double  monsters.  The  anadidymi  represent  the 
standard  type  of  vertebrate  double  monstrosity  and  it 
is  toward  their  explanation  that  most  of  the  theories  of 
the  past  have  been  directed. 


MODES  AND  CAUSES  OF  HUMAN  TWINNING     125 

THEORIES    OF   THE   MODE    OF   ORIGIN    OF 
HUMAN   DOUBLE   MONSTERS 

In  reviewing  Dareste's  work  on  the  origin  of  double 
monsters  in  the  chick  we  had  occasion  to  present  a  brief 
history  of  European  opinion  as  to  mode  of  origin  of 
human  double  monsters  (see  pp.  74-76).     These  vie 
need  not  be  restated  here.     Suffice  it  to  say  that  the 
weight  of  opinion  was  in  favor  of  an  origin  by  fusion  of 
separate  embryos.     In  America,   however,  we  find   an 
early  expression  of  the  fission  theory.     In  1866  (..   II. 
Fisher  postulated   a   theory   that  human  double  mon- 
strosity is  due  to  an  early  total  fission  of  the  embryo, 
followed  by  a  subsequent  fusion  of  the  two  parts.     He 
says  that  double  monsters  "are  invariably  the  product 
of  a  single  ovum,  with  a  single  vitellus  and  vitelline 
membrane,   upon  which   a   double   cicatricula,   or   two 
primitive  traces  are  developed.     The  several  forms  of 
double  malformation,  the  degree  of  duplicity,  the  char- 
acter and  extent  of  the  fusion,  all  result  from  the  prox- 
imity and  relative  positions  of  the  neural  axes  of  the  two 
more  or  less  definite  primitive  traces  developed  on  the 
vitelline  membrane  of  a  single  ovum."     This  idea,  the 
reader  will  note,  implies  that  all  united  parts  of  double 
monsters  are  fusion  products,  a  view  quite  inadmissible 
in  view  of  the  various  facts  already  stated  and  that  at 
soon  to  be  discussed.     The  fission  idea  is  also  tar  from 
clear.     We  are  not  told  when  or  how  the  two  primitive 
traces  originate.     Fisher  uses  the  word  "fission"  loosely 
to  mean  some  sort  of  dividing  process  giving  rise  to  two 
embryonic  areas  on  one  egL,r.     He  has  no  conception  that 
even  remotely  resembles  that  involved  in  our  theory  of 
fission  which  has  been  several  times  stated. 


126  THE  PHYSIOLOGY  OF  TWINNING 

In  1904  Wilder  in  an  important  discussion  of  human 
duplicate  twins  and  double  monsters  proposes  the 
"blastotomy  theory"  of  such  duplicities.  His  idea, 
which  has  already  been  discussed  in  The  Biology  of  Twins 
and  need  only  be  mentioned  here,  is  that  separate  twins 
result  from  the  complete  separation  of  the  blastomeres 
of  the  two-cell  stage  of  the  ovum,  and  that  double 
monsters  result  from  incomplete  separation  of  these 
blastomeres.  The  degree  and  position  of  the  union 
between  these  twins  are  attributed  to  variations  in  the 
points  of  contact  of  the  two  cells.  If  they  remain 
attached  by  apical  ends,  we  would  have  Janus  monsters; 
if  by  the  basal  ends  we  would  have  pygopagi;  if  by 
ventral  sides,  thoracopagi.  Since  the  discovery  of  the 
mode  of  twinning  in  the  armadillos  Wilder  himself  has 
abandoned  his  view  in  favor  of  the  "budding  theory." 
If,  however,  the  budding  theory  turns  out  to  be  inade- 
quate for  the  armadillos,  there  is  even  less  reason  for 
its  adoption  in  the  case  of  human  one-egg  twins,  and 
especially  is  it  inapplicable  to  that  of  conjoined  twins. 

streeter's  theory  of  the  origin  of 
human  twins 

In  view  of  the  fact  that  no  really  early  cases  of  one- 
egg  twinning  are  known  for  man,  the  theory  has  pre- 
vailed that  the  process  must  be  closely  similar  to  that 
of  the  armadillo.  The  facts  that  in  both  man  and  the 
armadillo  the  uterus  is  simplex,  that  there  is  a  similar 
ectodermic  mass  and  subsequently  a  similar  ectodermic 
vesicle  involving  a  similar  method  of  amnion  formation, 
have  made  it  seem  highly  probable  that  twinning  in  man 
is  equivalent  to  the  first  step  in  twinning  in  the  armadillo 


MODES  AND  CAUSES  OF  HUMAN  TWINNING     127 

which  Patterson  has  called  "primary  budding,"  bul 
which  I  interpret  as  a  very  simple  sort  of  bilateral  fission 
determined  by  the  bilaterality  of  the  uterus.  If  we 
could  only  secure  an  early  normal  stage  in  man  equivalent 
to  the  first  fission  stage  in  the  armadillo  we  could  readily 
settle  the  question  as  to  whether  the  same  mode  of  one- 
egg  twinning  occurs  in  these  two  quite  different  mammal.-. 
G.  L.  Streeter  (1919)  has  recently  made  an  exhaustive 
study  of  a  very  early  human  one-egg  twin  embryo 
(the  Mateer  ovum)  which  he  thinks  throws  considerable 
light  on  the  question  before  us.  This  is  much  the 
earliest  stage  of  human  twinning  we  have  discovered  and 
deserves  our  careful  consideration. 

The  twin  embryos  are  markedly  different  in  size  and 
in  stage  of  development.  The  larger  one  (which  Streeter 
calls  the  primary  embryo)  apparently  lies  in  normal  rela- 
tion to  the  yolk  sac  and  placenta  (Fig.  49,  C,  p.  1 28).  "It 
is  in  the  presomite  stage  and  has  only  just  acquired  a  prim- 
itive groove."  The  smaller  (which  Streeter  calls  the  twin 
embryo)  is  in  a  stage  about  equivalent  to  that  of  the 
armadillo  just  prior  to  the  first  step  of  twinning  or  before 
the  embryonic  axis  is  definitely  established.  This 
smaller  embryo  is  so  abnormally  situated  with  refereE 
to  the  larger  embryo  and  to  the  placenta  that  it  probably 
never  could  have  attained  satisfactory  nutritive  relations. 
It  does  not  seem  likely,  therefore,  that  we  have  in  this  ca 
a  typical  instance  of  twinning  such  as  might  produce 
duplicate  twins.  In  addition  to  the  fact  that  the  smaller 
twin  is  so  obviously  abnormal,  the  twins  differ  in  other 
ways  from  what  must  be  the  normal  situation  in  duplicate 
twins.  Streeter's  twins  have  entirely  separate  amnia, 
and  if  the  small  twin  were  to  placentate  it  would  have  a 


128 


THE  PHYSIOLOGY  OF  TWINNING 


separate  placenta.  This  is  in  contrast  with  the  fact 
that  some  human  twins,  especially  double  monsters, 
have  a  common  amnion;  and  one-egg  twins  always  have 
a  common  discoid  placenta. 

On  the  basis  of  his  studies  of  this  embryo,  however, 
Streeter  proposes  a  theory  of  human  twinning  which  is 


Fig.  49. — Schematic  drawing,  showing  Streeter's  idea  of  the  forma- 
tion of  a  human  one-egg  twin.  The  stages  are  drawn  to  the  same  scale 
of  enlargement  so  that  they  may  be  directly  compared.  A.  Stage  corre- 
sponding to  the  Miller  specimen,  showing  a  hypothetical  twin  budding  off 
from  the  primary  embryonic  node.  B.  Stage  corresponding  to  the  Bryce- 
Teacher  specimen.  C.  The  Mateer  specimen.  The  relatively  small  size 
of  the  twin  in  this  specimen,  and  the  detachment  of  the  yolk  sac  from  the 
amniotic  vesicle  are  indications  of  arrest  in  development.  (From  Streeter.) 


MODES  AND  CAUSES  OF  HUMAN  TWINNING     129 

essentially  a  fission  theory  similar  to  that  proposed  by 
Assheton  for  his  early  sheep  twins.  Streeter  considers 
that  the  fission  process  takes  place  at  the  inner-cell-ma 
stage.  This  mass  or  "embryonic  node"  is  believed  to 
undergo  subdivision  into  two  more  or  less  equivalent 
masses.     If  the  two  masses  are  equal  in  size 

then  chances  of  developing  in  an  orderly  manner  would  be  equal, 
and  this  is  presumably  what  happens  in  most  instances  of  identical 
twins.  Where  the  secondary  bud  is  merely  a  fragment  of  the 
original  mass  we  would  expect  that  there  would  be  some  degree 
of  differentiation;  but  the  process  of  development  would  soon  be 
arrested,  and  at  term  the  stunted  bud  would  be  found  as  a  small 
epithelial  cyst  on  the  placenta  near  the  attachment  of  the  umbilical 
cord.  In  case  the  twin-bud  is  only  partially  detached  from  the 
primary  node  there  would  exist  the  basis  for  the  various  ty; 
of  double  monsters  and  teratoma. 

It  will  be  noted  that  Streeter  uses  the  language  of  the 
budding  theory,  probably  influenced  by  myself  and  by 
Patterson,  but  the  process  which  he  describes  and  figures 
cannot  rightly  be  called  budding,  especially  if  the 
embryonic  node  divides  into  two  equal  masses,  for  in 
this  case  we  could  hardly  speak  of  one  as  the  bud  and 
the  other  as  the  stock.  As  has  already  been  said,  the 
theory  in  no  way  resembles  the  budding  theory  of 
Patterson,  but  is  definitely  a  fission  theory.  The  chief 
objections  to  Streeter's  theory  are  that  it  fails  to  account 
for  the  symmetry  relations  of  duplicate  twins  and  for 
the  fact  that  such  twins  frequently  have  a  common 
amnion.  Moreover  double  monsters,  which  are  believed 
to  belong  to  the  same  series  of  twins,  always  have  a 
common  amnion  and  have  strikingly  symmetrical  and 
intimate  interrelations:  conditions  that  could  not  be 
accounted  for  unless   the   embryonic  axis   were   either 


130  THE  PHYSIOLOGY  OF  TWINNING 

formed  or  were  forming  during  the  twinning  process. 
The  type  of  twinning  described  by  Streeter  takes  place 
at  an  early  blastula  stage  and  should  be  classed  as  a 
case  of  fission  of  an  early  blastoderm  to  form  two  sepa- 
rate embryonic  primordia. 

arey's  theory  of  the  origin  of  human  twins 

Just  as  I  was  revising  the  manuscript  of  this  volume 
there  appeared  two  papers  by  Arey  (1922a,  19226),  de- 
scribing certain  early  human  one-egg  twins,  that  throw 
new  light  on  our  present  problem.  It  is  not  uncommon 
to  find  in  human  beings  cases  of  tubal  pregnancy.  One 
or  more  ova  are  fertilized  in  a  Fallopian  tube  and  because 
of  certain  pathological  conditions  remain  in  the  tube  and 
acquire  a  sort  of  makeshift  placentation.  Embryonic 
development  may  proceed  for  months  before  fetal  death 
occurs.  Arey  has  brought  together,  after  a  rigid  exami- 
nation of  the  literature,  some  sixty  cases  of  human  tubal 
twins,  about  two-thirds  of  which  are  monochorial.  In 
view  of  the  fact  that  uterine  monochorial  twins  (probably 
always  one-egg  twins)  are  only  about  one-fourth  as 
numerous  as  dichorial  twins,  it  appears  that  tubal  mono- 
chorial twins  are  eight  times  as  frequent  as  "  might  be 
expected  if  the  tube  were  no  more  favorable  than  the 
uterus  as  a  seat  for  twin  production." 

Arey  himself  describes  two  cases  of  monochorial  twins 
that  are  especially  significant: 

The  first  of  the  two  specimens  consisted  of  a  single  chorionic 
sac  which  contained  twin  embryos,  each  12.3  mm.  long.  There  is 
a  common  yolk  sac  from  which  distinct  yolk  stalks  arise  near 

together   and    pass   to    their   respective  umbilical  cords 

The  second  new  twin  specimen  is  in  some  respects  more  interest- 
ing.   Within  a  single  chorion  were  twin  embryos  of  11.5  and 


MODES  AND  CAUSES  OF  HUMAN  TWINNING 

12   mm.     Each  had   its   individual   umbilical    cord;    these    w< 
attached    to   the    chorionic    wall,    a    quadrant's    distance    apart. 
Adherent  to  the  amnion  of  one  embryo  was  a  yolk  sac  of  normal 
size The  other  embryo  has  no  yolk  sac. 

Arey  enters  into  a  discussion  as  to  the  bearings  of  tin- 
lack  of  yolk  sac  in  the  latter  twin,  which  seems  t<>  be  of 
little  value  for  our  theory.  Our  chief  concern  has  to  do 
with  the  mode  of  origin  of  these  two  cases  of  human 
twins.  The  first  case  is  almost  certainly  a  case  of  double 
gastrulation  of  a  distinctly  s>Tnmetrical  sort  like  those 
of  the  starfish  shown  in  Figures  4-6  or  like  certain  chick 
twins  such  as  that  in  Figure  36.  The  mode  of  origin 
of  the  other  twin  embryo  is  uncertain.  Since  the  two 
embryos  are  not  attached  to  the  same  yolk  sac  they  haw 
probably  originated  from  an  early  total  fission  of  the 
blastoderm  or  embryonic  node  much  like  the  hypothetical 
case  of  Streeter  except  that  the  fission  must  have  resulted 
in  two  practically  equal  primordia  both  of  which  wen- 
able  to  form  a  placenta. 

MODES   OF  HUMAN   ONE-EGG   TWINNING 

Although  the  evidence  is  still  somewhat  meager  we 
are  now  in  a  position  to  state  with  some  confidence  that 
the  same  three  modes  of  one-egg  twinning  occur  in  the 
case  of  man  as  have  been  described  for  previous  inverte- 
brate and  vertebrate  types:  (a)  twins  produced  by 
fission  of  the  blastoderm,  as  illustrated  by  Streetei 
case  and  the  second  case  of  Arey;  (b)  twins  produced 
by  double  gastrulation,  as  in  Arey's  firsl  case; 
double  monsters,  and  possibly  some  entirely  separate 
twins,  produced  by  partial  or  complete  fission  of  the 
bilateral  halves  of  a  single  embryonic  axis. 


132  THE  PHYSIOLOGY  OF  TWINNING 

THE   CAUSES    OF   TWINNING   IN  MAN 

The  most  nearly  direct  evidence  bearing  on  the  cause 
of  monozygotic  twinning  in  man  is  derived  from  certain 
data  presented  by  Arey  (19226),  already  referred  to 
above.  He  has  shown  that  monochorial  twins  are  many 
times  as  numerous  in  the  Fallopian  tubes  as  in  the  uterus. 
The  tubes  are  far  from  being  a  normal  locality  for  the 
placentation  of  the  embryo  and  there  is  reason  to  believe 
that  even  the  makeshift  placentation  that  does  take 
place  is  greatly  belated.  If  this  assumption  be  warranted 
we  have  a  situation  quite  similar  to  the  "  period  of 
quiescence"  in  the  armadillo,  and  the  consequence  would 
be  the  same:  partial  loss  of  axiate  organization  and 
a  physiological  isolation  of  two  secondary  apical  points 
or  points  of  gastrulation. 

Thus  we  might  be  able  to  account  for  monochorial 
twinning  in  the  Fallopian  tubes,  but  we  would  still  have 
to  explain  uterine  monochorial  twinning.  The  nearest 
approach  to  direct  evidence  of  the  causes  of  uterine 
monochorial  twinning  is  furnished  by  Stockard  (192 1). 
A  case  of  triplets  came  to  his  attention  in  which  one 
individual  was  born  a  normal  female  baby. 

After  delivering  the  child  the  physician,  Dr.  Erdwurm,  noted 
that  a  second  chorionic  sac  ruptured  and  discharged  its  fluid. 
Later  two  dead  twin  female  fetuses  were  delivered.  These  lay 
in  a  common  amnion  with  their  umbilical  cords  twisted  around 
one  another  in  such  a  way  that  they  had  probably  cut  off  both 
blood  connections. 

In  further  comment  on  this  case,  Stockard  goes  on  to  say: 

My  interpretation  of  this  triplet  condition  is  as  follows: 
The  mother  liberated  from  the  ovary  two  eggs,  both  of  which 
became  fertilized  and  began  development.     One  became  implanted 


MODES  AND  CAUSES  OF  HUMAN"  TWINNING     133 

slightly  before  the  other  and  developed  into  the  single  living  girl. 
The  second  egg  was  not  so  favorably  implanted  as  the  first;  this 
is  indicated  in  the  specimen  by  the  lower  placenta  riding  upon  the 
larger  one.  The  delay  in  implantation,  due  to  the  presem  e  of  the 
first  egg,  caused  a  slow  rate  of  development  at  an  earl)  in 

the  second  and  two  embryonic  buds  arose  instead  of  one,  just 
was  described  on  the  germ  ring  of  the  fish.    In  this  human  specimen 
there  is  fortunately  present  the  physical  cause  that  might  have 
produced  the  delay. 

This  explanation  of  Stockard's  has  unfortunately  a 
very  limited  application,  for  it  is  extremely  rare  that  one- 
egg  twins  occur  along  with  another  embryo.  As  a  rule 
the  egg  destined  to  produce  twins  has  the  whole  uterus 
to  itself;  it  could  not  be  retarded  by  the  prior  placenta- 
tion  of  another  egg.  We  must  therefore  look  elsewhere 
for  probable  retarding  agencies.  Three  possible  retard- 
ing factors  seem  possible: 

1.  Under  stimulation  of  the  egg,  due  to  some  deject  in 
the  development-initiating  mechanism  of  the  sperm.  Dav- 
enport has  shown  that  twinning  is  rather  strongly  inher- 
ited in  the  male  line.  If  this  be  the  case  it  could  hardly 
affect  two-egg  twinning,  since  this  is  a  phenomenon  of 
ovulation  and  concerns  only  the  female.  It  would 
seem  then  that  only  one-egg  twinning  could  be  affected 
through  the  male  line.  If  the  egg  were  retarded  through 
insufficient  stimulation  on  the  part  of  the  sperm  it  would 
probably  undergo  belated  fission,  the  consequences  of 
which  would  depend  upon  the  degree  of  retardation. 

2.  Belated  placentation,  due  to  a  failure  of  the  corpus 
luteum  to  stimulate  the  uterine  mucosa.     This  condition 
merely  implies   some  physiological    d incoordination   1> 
tween    the   various    intricately    interdependent    fact 
responsible  for  implantation  of  the  ovum.    The  weakni 


134  THE  PHYSIOLOGY  OF  TWINNING 

of  this  view  is  that  the  same  mechanism  would  presum- 
ably persist  throughout  the  reproductive  life  of  a  given 
mother  and  she  should  always  produce  twins.  Such  a 
condition,  however,  does  not  prevail,  for  almost  without 
exception  mothers  of  one-egg  twins  have  also  single 
children.  It  is  barely  possible,  however,  that  cases  of 
single  offspring  from  parents  exhibiting  one-egg  twinning 
are  not  true  single  offspring  but  that  each  is  the  survivor 
of  a  pair  of  twins,  one  of  which  has  succumbed  to  the 
ever-present  hazards  that  prevail  especially  in  connec- 
tion with  one-egg  twinning.  This  particular  explana- 
tion is  the  one  that  was  adopted  for  the  armadillo  case, 
where  prenatal  mortality  is  extremely  low.  On  the  whole 
this  seems  the  least  objectionable  causal  theory  of 
twinning  in  man. 

3.  A  third  possibility  is  that  twinning  is  a  hereditary 
character  dependent  upon  a  recessive  gene.  The  effect 
of  this  gene  would  have  to  be  thought  of  as  an  unfavor- 
able growth-retarding  factor  that  causes  a  temporary 
" period  of  quiescence"  like  that  in  the  armadillo, 
resulting  in  belated  placentation  and  twinning.  The 
cause  of  twinning,  according  to  this  theory,  is  purely 
intrinsic,  unaffected  by  environment,  and  could  be  as 
readily  transmitted  through  the  sperm  as  through  the 
egg.  If  two  individuals  heterozygous  for  the  twinning 
gene  mated,  some  of  the  zygotes  would  be  homozygous 
for  the  character  and  twinning  would  result.  This  theory 
would  account  for  the  fact  that  in  twinning  families  there 
may  be  some  single  offspring.  This  genetic  theory  of 
twinning  seems  to  me  on  the  whole  somewhat  fantastic, 
but  it  can  hardly  be  excluded  as  one  of  the  possibilities, 
especially  in  view  of  Davenport's  discovery. 


CHAPTER  X 

DEVELOPMENTAL  HAZARDS  OF  HI' MAN  TWIN- 
SEPARATE   TWINS 

ON   THE   INFLUENCES   WHICH   TWINS,    ESPECIALLY   ONE-EGG    TW11 
EXERT   UPON   EACH   OTHHK    IN    THE    UTERI   - 

A  popular  impression  prevails  that  in  human  twins 
one  is  usually  stronger  and  more  vigorous  than  the  other. 
Observations  of  the  writer  and  of  others  who  have  inter- 
ested themselves  in  these  matters  tend  to  bear  out  this 
impression.  Even  in  the  case  of  so-called  duplicate  or 
identical  twins,  the  products  of  a  single  egg.  there  is 
nearly  always  a  more  vigorous  twin  who  is  the  dominant 
member  of  the  combination.  There  is  also  a  somewhat 
vaguely  expressed  feeling  among  families  in  which  twin- 
ning has  occurred  that  one  twin  has  in  some  way  drawn 
upon  the  vitality  of  the  other  or  has  inherited  more  than 
his  fair  share  of  certain  essential  qualities,  leaving  the 
other  somewhat  depleted  in  energy  and  vigor,  A  more 
definite  form  of  this  type  of  idea,  to  wit,  that  one  twin 
is  commonly  sterile,  has  come  to  me  several  times  of 
late.  This  idea  may  have  had  its  origin  in  the  fr< 
martin  situation  among  cattle,  where  a  female  calf  burn 
twin  to  a  male  is  nearly  always  sterile.  The  possibility 
that  human  freemartins  may  occur  has  never  been 
adequately  affirmed  nor  denied. 

It  is  my  belief  that  these  popular  impressions  are  not 
without  foundation.     There  is  abundant  evident  :>e 

daily  in  the  case  of  one-egg  twins,  that,  as  the  direct 

135 


136  THE  PHYSIOLOGY  OF  TWINNING 

result  of  the  twinning  relation,  one  twin  tends  to  gain  a 
physiological  ascendancy  over  the  other,  to  the  slight 
or  very  great  detriment  of  the  latter.  As  to  the  extent 
to  which  one  twin  may  harm  the  other  during  pregnancy 
there  is  considerable  difference  of  opinion. 

Spaeth  (i860)  was  probably  the  earliest  observer  to 
study  this  problem.  He  was  chiefly  interested  in  the 
question  whether  the  interinfluence  between  fetuses  was 
greater  in  one-egg  than  in  two-egg  twins.  His  material 
consisted  of  sixty-five  pairs  of  new-born  twins  and  their 
embryonic  membranes.  Whether  the  twins  were  the 
products  of  a  single  egg  or  of  two  eggs  was  judged  by  the 
relations  they  bore  to  the  placenta  and  the  other  mem- 
branes, especially  the  amnion.  A  comparison  between 
one-egg  and  two-egg  twins  showed  that  twins  of  both 
kinds  are  nearly  always  rather  markedly  unequal  in 
size  and  in  body  length.  In  only  three  cases  out  of  the 
sixty-five  examined  were  the  twins  even  of  similar  size 
and  length.  One  of  these  cases  of  striking  similarity, 
judged  by  their  possession  of  a  common  placenta,  com- 
mon chorion,  and  common  amnion,  was  undoubtedly  a 
case  of  one-egg  twins.  The  other  two  cases  of  close 
similarity  were  in  two-egg  twins.  There  was  no  evi- 
dence that  the  twins  of  either  type  had  any  definite 
physiological  effect  upon  each  other  and  Spaeth  concludes 
that,  although  twins  are  so  closely  associated  during 
pregnancy,  they  maintained  a  high  degree  of  independ- 
ence. In  only  one  respect  does  he  see  evidences  of  inter- 
influence: in  the  occurrence  of  situs  inversus  viscerum. 
In  a  number  of  cases  he  noticed  in  one  of  the  twins  a 
reversed  symmetry  of  stomach,  heart,  liver,  and  other 
more  or  less  asymmetrical  organs.     As  the  whole  ques- 


DEVELOPMENTAL  HAZARDS  OF  HI  M  \\    HVINS    i.,; 

tion  of  symmetry  in  twins  is  discussed  in  a  lain-  chapti 
of  this  book,  we  may  postpone  for  the  present  a 
ment  of  Spaeth's  opinions  on  this  subject. 

Schatz,  who  has  written  more  extensively  than  any 
other  writer  about  human  one-egg  twins,  hold-  quite  a 
different  opinion  from  that  of  Spaeth  as  to  the  influeni 
of  twins  upon  each  other.  This  author  had  the  advan- 
tage of  an  adequate  mass  of  data:  an  admirable  collec- 
tion of  twin  embryos  and  fetuses,  together  with  their 
fetal  membranes,  from  the  Marburg  and  Rostock  gyne- 
cological clinics.  No  other  body  of  data  on  human  twins 
comparable  to  this  has  ever  been  brought  together.  In 
several  extensive  tables  Schatz  gives  lengths  and  weights, 
together  with  percentage  differences  in  weights  and 
lengths  of  twins.  These  are  put  into  groups  based  on 
the  length  and  weight  of  the  larger  twin.  Two  clas 
of  twins  are  distinguished: 

A.  Two-egg  twins 

i.  Twins  in  which  the  two  placentae  arc  entirely  separate 
2.  Twins  in  which  the  two  placentae  are  more  or  loss  fused 

B.  One-egg  twins  (always  with  but  one  placenta) 

The  abundance  of  material  enables   the   author   t<> 
compare  the  developmental  differences  of  the  two  cl 
of  twins  at  various  periods  of  pregnancy,  instead  of  only 
after  birth,  as  Spaeth  had  done.     This  method  reveals 
the  following  important  facts: 

a)  The  differences  between  two-egg  twins,  irrespec 
tive  of  whether  or  not   the  placentae   are   separate   or 
fused,  increase  steadily  up  to  and  after  birth. 

b)  The  differences  between  one  egg  twins  are  great< 
at  about  the  middle  of  pregnancy  and  d<  adily 
until  or  after  birth. 


138  THE  PHYSIOLOGY  OF  TWINNING 

c)  The  result  is  that  the  two  types  of  twins  show 
about  equal  differences  at  birth,  a  fact  which  is  in  agree- 
ment with  Spaeth's  findings. 

d)  At  birth  one-egg  #twins  after  a  long  period  of 
decreasing  difference,  and  two-egg  twins  after  a  still 
longer  period  of  increasing  difference,  come  to  a  period 
of  approximate  equality.  It  follows  from  this  that 
during  the  whole  period  of  pregnancy  one-egg  twins  are 
distinctly  more  different  than  are  two-egg  twins.  And 
this  is  a  more  striking  circumstance  in  view  of  the  fact 
that  their  origin  from  one  egg  should  tend  to  make  them 
more  alike  rather  than  more  different. 

e)  The  only  conclusion  to  be  derived  from  these  facts 
is  that  the  conditions  of  one-egg  twinning  tend  to  cause 
one  twin  to  have  a  pronounced  effect  upon  the  develop- 
ment of  the  other.  Schatz  has  made  an  exhaustive 
study  of  the  ways  in  which  one  twin  may  influence  the 
other. 

THE   DISADVANTAGES    OF   TWINNING 

Before  entering  upon  an  account  of  the  ways  in  which 
human  one-egg  twins  influence  each  other's  develop- 
ment, let  us  consider  briefly  some  of  the  general  dis- 
advantages of  twinning  over  single  births.  The  human 
uterus  is  of  the  simplex  or  undivided  type  and  is  adapted 
for  the  really  satisfactory  gestation  of  but  one  fetus  at 
a  time.  When  two  or  more  fetuses  come  to  occupy  the 
space  usually  filled  by  one,  the  twins,  whether  of  the 
one-egg  or  two-egg  type,  crowd  each  other  and  compete 
for  the  common  food  supply.  In  the  case  of  two-egg 
twins  it  probably  often  happens  that  one  egg  reaches  the 
region  of   attachment  first   and   tends   to   occupy   the 


DEVELOPMENTAL  HAZARDS  OF  HUMAN    [WINS    I 

available  area  to  the  complete  or  partial  exclusion  of 

the  other.  Stockard  describes  one  cu.m-  which  he  inter- 
prets in  that  way,  in  which  the  later  egg,  failing  to 
gain  a  good  placental  attachment,  underwent  twinning, 
probably  as  the  result  of  retarded  development.  Sub- 
sequently at  about  the  middle  of  pregnancy  the  twin 
embryos  died  through  a  complete  shutting  off  of  nutri- 
tion, while  the  original  single  fetus  went  on  to  full  term. 
It  seems  probable  then  that  the  main  influences  exercised 
by  two-egg  twins  upon  each  other  are  the  result  of  com- 
petition for  placental  surface.  In  addition  to  the  extra 
hazards  due  to  competition,  twins  seem  to  fall  heir  to 
all  of  the  ordinary  hazards  met  with  by  single  fetusi 
such  as  loosened  placenta,  twisted  and  knotted  umbilical 
cord,  stricture  or  breaking  of  umbilical  blood  vessels, 
rupture  of  amnion,  loss  of  amniotic  iluid.  and  the  result- 
ant adhesions.  The  period  of  uterine  gestation  is  at 
best  a  hazardous  one,  but,  quite  in  addition  to  all  of  the 
hazards  that  are  met  by  single  embryos  and  those  that 
are  shared  also  by  two-egg  twins,  there  are  certain  very 
serious  special  dangers  that  fall  upon  one-egg  twins  by 
reason  of  their  close  genetic  relationship. 

THE  SPECIAL  HAZARDS  OF  ONE-EGG    rWINS 

For  the  data  herewith  presented  1  am  indebted  to  the 
numerous  contributions  to  our  knowledge  of  the  develop- 
mental physiology  of  one-egg  twins  by  Friedrich  Schal 
These  papers  all  appeared  in  the  Archiv  fiir  C  >lo- 

gie  between  the  years   [882  and    k;oo.      This  author  had 
the  advantage  of  studying  abundant  material  well  pi 
served  and  adequately  injected.     No  question  seems  to 
exist  in  his  mind  as  to  the  occurrem  e  of  human  on< 


140  THE  PHYSIOLOGY  OF  TWINNING 

twins  and  there  seems  to  be  no  difficulty  in  distinguishing 
one-egg  from  two-egg  twins.  A  study  of  the  detailed 
anatomy,  especially  the  vascular  anatomy  of  twin 
fetuses,  together  with  that  of  the  fully  injected  placen- 
tae and  umbilical  blood  vessels,  enables  the  author  to 
determine  the  probable  mechanism  of  the  interirinuences 
of  one-egg  twins.  Leaving  out  of  consideration  all  injuri- 
ous conditions  which  one-egg  twins  may  have  in  common 
with  two-egg  twins  or  with  single  fetuses,  let  us  focus 
our  attention  upon  those  interinnuences  peculiar  to  one- 
egg  twins. 

Schatz  distinguishes  two  types  of  fetal  interinfluence 
incident  to  one-egg  twinning.  The  first  is  associated 
with  situs  inversus  viscerum  or  the  possession  by  one  of 
the  twins  of  an  asymmetry  of  the  heart,  stomach,  and 
viscera  which  is  the  mirror-image  of  that  of  the  other 
twin  or  of  that  characteristic  of  the  species.  This 
reversal  is  conceived  of  as  a  direct  result  of  the  twinning 
process,  though  there  is  no  definite  theory  to  account 
for  it.  All  degrees  of  inverse  symmetry  are  noted, 
ranging  between  a  slight  degree  of  it  to  complete  reversal. 
It  is  very  common  in  conjoined  one-egg  twins  and 
relatively  rare  in  separate  one-egg  twins.  Schatz  con- 
siders that  inverse  symmetry  of  the  blood  vessels, 
especially  when  the  reversal  is  slight  or  incomplete,  has 
very  serious  consequences  for  the  unfortunate  twin  in 
which  the  inverse  symmetry  exists.  Among  other  things, 
it  may  lead  to  a  bad  connection  with  the  umbilical  blood 
supply,  which  remains  normal  in  its  relations.  Schatz 
enters  into  a  detailed  discussion  of  the  secondary  effects 
upon  the  vascular  system  of  a  fetus  in  which  inverse 
symmetry  exists  and  cites  a  number  of  cases  of  badly 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWINS    141 

deformed  twins  which  are  interpreted  as  the  result  of 
such  an  original  inverse  symmetry.  It  is  interesting  to 
note  that  symmetry  reversal,  besides  being  diagnosed 
as  a  definite  criterion  of  one-egg  twins,  is  considered  as 
a  hazard  of  twinning.  Further  discussion  of  rever 
symmetry  is  to  be  found  in  chapter  xii. 

INTERINFLUENCES   OF   SEPARATE   ONE-EGG    TWINS 

Human  one-egg  twins  have  a  common  discoid 
placenta  to  which  are  attached  the  two  umbilical  cords. 
As  a  rule  the  two  cords  are  symmetrically  placed  upon 
the  placenta,  though  there  are  some  noteworthy  cases 
of  asymmetrical  attachment.  Sometimes  the  two  cords 
are  near  together  in  the  center  of  the  placenta;  sometimes 
they  are  on  opposite  sides  and  near  the  margin.  There 
is  even  some  evidence  that  the  attachment  of  the  two 
cords  more  or  less  closely  coincides  with  the  right  and 
left  sides  of  the  uterus,  reminding  one  of  the  situation 
in  the  armadillo.  Whatever  may  be  the  point  of  attach- 
ment of  the  two  cords  on  the  single  placental  area,  the 
twins  divide  this  area  more  or  less  equally  between  them. 
There  is  opportunity  for  competition  here;  for  the  twins 
may  develop  at  slightly  different  rates  and  the  one  that 
first  develops  a  placenta  is  likely  to  acquire  more  than 
its  fair  share  of  placental  area,  and  hence  more  than  half 
of  the  available  nutriment.  This  may  account  for  a 
part  of  the  marked  size  difference  between  one  eurur  twins 
during  the  middle  period  of  pregnancy, 

A  far  more  important  condition  leading  to  interin 
fluence  arises  probably  as  a  dire*  t  resull  ol  a  competition 
for   placental   area.     In    the   /one    of    competition    the 
separate  placental  circulations  of  the  twins  come  very 


142  THE  PHYSIOLOGY  OF  TWINNING 

closely  into  contact  and  more  or  less  extensive  anasto- 
moses of  capillaries,  arteries,  and  veins  take  place  between 
the  two  circulations.  Four  types  of  vascular  inter- 
communication are  distinguished  in  these  twin  placentae : 

A.  In  almost  all  one-egg  twins  there  occur  in  the 
competitive  zone  twenty  or  more  villous  trees  which  are 
occupied  in  common  by  the  circulations  of  the  twins. 
The  arteries  of  one  twin  occupy  half  of  such  a  villous 
tree  and  the  veins  of  the  other  twin  occupy  the  other  half. 
The  real  connection  between  the  circulations  here  is 
through  capillaries  only. 

B.  In  addition  to  the  villous  transfusion  there  may 
exist  cases  with  one  or  more  superficial  arterial  anas- 
tomoses. 

C.  Instead  of  arterial,  there  may  be  one  or  more 
venous  anastomoses. 

D.  Many  placentae  show,  in  addition  to  villous 
transfusion,  both  venous  and  arterial  anastomoses. 

Types  A  and  C  are  rare;  types  B  and  D  are  frequent. 
In  brief,  there  are  nearly  always  at  least  superficial 
arterial  anastomoses,  either  with  or  without  compensat- 
ing venous  anastomoses.  This  region  of  intercommuni- 
cation between  the  placental  vascular  systems  constitutes 
what  Schatz  calls  the  third  circulation.  This  third  circu- 
lation has  a  volume  only  about  one-tenth  or  even  one- 
twentieth  as  great  as  that  of  the  general  circulation  of 
one  twin.  Small  in  volume  as  this  may  be,  much  of  the 
welfare  of  the  twins  hinges  upon  whether  it  is  symmetrical 
or  asymmetrical.  If  it  is  symmetrical  with  reference  to 
the  volume  of  blood  exchanged  between  the  twins,  all 
is  well  with  both  twins;  but  if  the  arterial  contribution 
of  one  is  greater  than  that  of  the  other,  or  the  venous 


DEVELOPMENTAL  HAZARDS  OI    111  M\\    rWINS    143 

contribution  of  one  is  less  than  that  of  the  other,  a  serious 
situation  is  sure  to  arise  and  the  degree  of  seriousni 
depends  on  the  degree  of  asymmetry.  Which  of  the 
twins  in  any  case  is  the  more  damaged  depends  upon  the 
particular  conformation  of  the  asymmetry.  The  usual 
result  of  placental  anastomosis  is  that  the  amount  of 
blood  which  flows  from  the  first  twin  to  the  second  i- 
not  entirely  the  same  as  that  which  flows  from  the  >nd 
to  the  first.  There  therefore  exists  in  most  twin  placentae 
a  dynamic  asymmetry  of  the  third  circulation  which  is 
not  equalized  by  venous  anastomoses  and  must  t hen- 
fore  be  equalized  by  functional  adjustments  in  the  bodies 
of  the  twins. 

Figures  50  and  51  (pp.  144,  145)  represent  typical 
placental  relations  in  separate  one-egg  twin-.  In  both 
there  is  a  decided  asymmetry.  In  Figure  50  the  twin 
A  is  favored  by  the  circulation  and  in  Figure  51  the 
twin  B  is  favored. 

Although  the  word  " favored''  has  been  used  we  must 
understand  that  this  term  is  merely  relative,  for  it  would 
be  much  better  for  both  twins  if  no  anastomoses  of  their 
placental  circulations  occurred  or  if  the  balance  of 
exchange  were  equalized  within  the  placental  circulation 
itself.  It  is  somewhat  more  immediately  harmful  for 
a  twin  to  be  robbed  of  part  of  its  blood  than  to  gain  a 
constant  access  of  blood  from  the  other  twin;  but  I 
much  blood  is  in  the  end  decidedly  harmful.  It  we  may 
speak  of  the  twin  which  gains  additional  blood  through 
the  asymmetry  of  the  third  circulation  as  the  red 
twin  and  the  twin  which  loses  blood  as  the  injured  twin, 
we  may  discuss  separately  the  effects  ol  these  disturb 
ances  in  the  two  kinds  of  twins. 


144 


THE  PHYSIOLOGY  OF  TWINNING 


THE  INFLUENCE  OF  ASYMMETRY  OF  THE  THIRD  CIRCULATION  UPON 
THE   DEVELOPMENT   OF   THE    FAVORED   TWIN 

The  primary  effect  of  excess  blood  is  naturally  pleth- 
ora or  an  overfulness  of  the  vessels.     The  consequences 


Fig.  50. — -The  common  placenta  and  the  hearts  of  a  typical  pair  of 
one-egg  human  twin  fetuses.  Note  the  symmetrical  arrangement  of  the 
umbilical  cords,  the  superficial  intertwin  anastomoses  of  placental  blood 
vessels  within  the  dotted  areas.  The  arteries  are  stippled  on  the  left, 
unshaded  on  the  right.  The  veins  are  cross-hatched  on  the  left  and  solid 
black  on  the  right.  The  heart  of  the  left  twin  (below)  is  greatly  reduced, 
that  of  the  right  is  decidedly  enlarged.  The  left-hand  twin  is  the  so- 
called  "injured"  one  and  the  right-hand  twin,  the  so-called  "favored" 
one.     See  text  for  further  explanation.     (After  Schatz.) 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWINS    [45 

of  plethora  are:  first,  a  general  development  more  Dearly 
normal  than  that  of  the  injured  twin;  second,  a  height- 
ened blood  pressure  in  the  venous  system.  Some 
secondary  results  are:   a  general  oedematous  and  drop- 


Fig.  51. — Another  typical  common  placenta  of  ;i  pair  of  one  - 
human  twins  in  which  the  vascular  anastomoses,  forming  the  so-called 
"third  circulation"  is  extensive,  though  not  greatly  unbalanced.     Even 
this  degree  of  imbalance  had  very  serious  consequences  on  both  twins. 
(After  Schatz.) 

sical  condition  of  the  body,  the  umbilical  cord,  and  the 
placenta;  hypertrophy,  followed  by  atrophy,  of  the  liver; 
more  or  less  marked  hypertrophy  <>!"  the  entire  heart. 
which  becomes  not  only  relatively  Larger  than  that  of 
the  injured  twin,  but  positively  Larger  than  those  of 
single  fetuses  of  similar  size;  hypertrophy  of  the  Left 
ventricle,    observable    in    new-born    twins:     heightened 


146  THE  PHYSIOLOGY  OF  TWINNING 

arterial  blood  pressure;  intra-uterine  opening  up  of  the 
pulmonary  circuit;  thickening  of  the  walls  of  the  blood 
vessels,  especially  of  the  arteries;  hypertrophy  of  kid- 
neys and  bladder;  excessive  urine;  and  excessive  am- 
niotic fluid. 

All  of  these  changes  are  decidedly  detrimental  and 
have,  in  most  cases,  caused  the  premature  death  or 
abortion  not  only  of  the  injured,  but  of  the  favored 
twin.  Only  those  cases  in  which  a  marked  difference 
between  the  twins  existed  have  been  made  the  object  of 
special  study;  yet  one  cannot  help  but  suspect  that,  even 
in  twins  that  are  nearly  equal,  go  to  full  term,  and  live 
for  a  considerable  time  after  birth,  some  of  the  after- 
effects of  minor  degrees  of  the  changes  listed  above  may 
persist  in  one  or  both  twins  and  help  to  account  for  the 
lower  vitality  and  earlier  death  of  one  twin  or  the  pre- 
mature death  of  both  twins. 

Serious  as  are  the  effects  of  blood  exchange  for  the 
favored  twin,  they  are  trifling  as  compared  with  those  of 
the  injured  twin. 

THE  INFLUENCE  OF  ASYMMETRY  OF  THE  THIRD    CIRCULATION  UPON 
THE   DEVELOPMENT   OF   THE   INJURED   TWIN 

The  primary  effect  upon  the  injured  twin  is  a  diminu- 
tion of  the  blood  supply.  In  cases  of  only  slight  inequal- 
ity it  is  likely  that  a  somewhat  lessened  blood  supply 
is  a  healthier  condition  than  one  somewhat  increased. 
The  after-effects  are  not  so  likely  to  be  serious.  In 
cases  of  marked  inequality,  however,  the  consequences 
are  for  the  injured  twin  very  serious  indeed,  because 
nutrition  is  so  reduced  that  development  is  either 
entirely  arrested  or  certain  particular  structures  fail  to 
emerge  from  the  fetal  condition.     In  less  pronounced 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWINS    [47 

cases  development  is  at  least  measurably  retarded.  The 
diminished  blood  pressure  and  the  lack  of  turgesceni 
are  often  followed  by  definite  nutritional  disturbance 
The  kidneys,  through  lessened  functioning,  are  arrested 
and  as  a  consequence  there  is  a  decrease  in  tin-  se<  retion 
of  urine  and  a  corresponding  resorption  of  the  amniotic 
fluid.  This  may  be  followed  by  serious  mechanical 
consequences  such  as  pressure  of  the  fetus  against  the 
membranes  and  consequent  adhesions.  As  a  result  of 
these  physiological  disturbances,  intra-uterine  death 
often  occurs  in  one  of  the  twins.  Schatz  has  shown 
that  intra-uterine  death  in  one-egg  twins  occurs  tin 
times  as  frequently  as  in  two-egg  twins.  This  scums  t<> 
indicate  that  a  high  percentage  of  the  prenatal  deaths  of 
one-egg  twins  is  to  be  attributed  to  the  influences  such 
twins  exert  upon  each  other  through  their  intimate 
vascular  interrelations.  Probably  at  least  half  of  tin- 
deaths  in  one-egg  twins  are  due  to  causes  that  are  inde- 
pendent of  twinning,  such  as  primary  death  of  the  heart 
or  twisting  of  the  umbilical  cord.  The  remaining  death- 
are  those  in  which  we  are  especially  interested  because 
they  are  due  to  conditions  peculiar  to  one-egg  twins: 
namely,  pronounced  asymmetry  of  the  placental  third 
circulation  and  the  immediate  or  secondary  consequent  es 
of  the  latter. 

HOW  THE   DAMAGE    IS    D»>\i 

As  the  result  of  an  anatomical  derangement  or  asym- 
metry of  the  vascular  system,  one  of  the  twin-  i-  robbed 
of  the  blood  supply  necessary  for  it s  normal  nourishment 
and  functioning.  The  result  is  a  progressive  weakening  ol 
the  heart  with  an  accompanying  decrease  in  si  The 
pressure  of  the  blood  from  the  stronger  opposite  twin 


148  THE  PHYSIOLOGY  OF  TWINNING 

comes  to  bear  upon  this  weakened  heart  and,  if  suffi- 
ciently strong,  overwhelms  it  and  brings  its  rhythm  to 
a  standstill.  Heart-death  ensues  as  the  direct  result  of 
its  relation  with  the  other  twin.  This  is  secondary 
heart-death.  Heart-death  from  any  other  cause  is 
primary  heart-death  and  is  not  a  consequence  of  twin- 
ning. 

A   HEART-DEAD   TWIN   KEPT   ALIVE   BY   ITS   PARTNER 

While  on  the  one  hand  a  twin  may  injure  its  partner 
by  robbing  it  of  its  blood  supply  and  suppressing  its 
heart  rhythm,  a  twin  which  has  suffered  heart-death  on 
its  own  account,  may,  on  the  other  hand,  be  kept  alive 
by  its  normal  partner.  This  life-saving  act  is  made 
possible  by  means  of  the  third  circulation.  Were  it  not 
for  extensive  blood  transfusion  between  the  normal  and 
the  heart-dead  twin  the  latter  would  die  and  disintegrate 
at  once.  Instead,  the  heart-dead  twin  is  kept  under  at 
least  partial  circulation  so  that  more  or  less  of  the  body 
continues  to  develop.  A  completely  normal  individual 
cannot,  however,  be  thus  reared;  for,  in  the  first  place, 
a  dead  heart  cannot  revive  and  therefore  atrophies,  and, 
in  the  second  place,  the  circulation  through  the  placenta 
from  the  normal  twin  is  never  sufficiently  abundant  nor 
energetic  to  afford  nutriment  to  all  parts  of  the  body. 
Usually  those  organs  situated  on  the  outskirts  of  the 
zone  of  circulation  are  the  first  to  be  deprived  of  their 
needed  share  and  are  arrested  in  their  development  or 
become  secondarily  resorbed.  For  these  reasons  the 
prolonging  of  life  in  the  dependent  twin  is  of  no  value 
either  to  it  or  to  the  normal  twin.  In  fact  the  vicarious 
heart  labor  consequent  upon  the  maintenance    of  an 


DEVELOPMENTAL  HAZARDS  OF  IHWIW   TWINS     [49 

additional    circulation    induces    heart    hypertrophy    in 
the  stronger  twin,  and   this  may    have   serious   cons 
quences  after  birth   when  the  extra   burden   has   been 
removed. 

Whether  one  twin  injures  the  other  or  prolongs  the 
life  of  the  other  we  find  that  the  most  striking  physio- 
logical and  anatomical  conditions  are  those  that  concern 
the  heart.  The  heart  dies  and  atrophies  completely,  it 
becomes  weak  and  small  in  size  or  becomes  overly  strong 
and  too  large  in  size.  We  shall  now  consider  the  various 
heart  anomalies  in  twins. 

HEARTLESS    TWINS    (ACARDIl) 

The  general  term  acardia  is  used  to  designate  a  con- 
dition common  in  twins,  especially  one-egg  twins,  charac- 
terized by  atrophy  of  the  heart.  Schatz  distinguishes 
between  complete  lack  of  heart  (holoacardia)  and  partial 
lack  of  heart  (hemiacardia).  In  a  twin  exhibiting  holo- 
acardia  the  circulation  is  carried  on  entirely  by  means  of 
the  heart  of  the  other  twin.  In  a  twin  with  hemiacardia 
the  circulation  is  carried  on  partly  by  the  foreign  heart 
and  partly  by  its  own  heart.  So  long  as  a  twin's  own 
heart  continues  to  function  to  any  extent,  or  even  if  the 
foreign  heart  is  only  locally  effective,  as  would  be  the 
case  if  the  direction  of  the  blood  stream  were  leversed 
in  an  umbilical  artery,  the  condition  would  be  diagnosed 
not  as  holoacardia  but  as  hemiacardia. 

There  may  be  as  many  as  twenty-eight  different 
situations  depending  upon  which  of  the  tour  types  oi 
placenta,  A, B, C,D  (seep.  142.'.  are  concerned  and  which 
one  of  the  following  seven  types  of  vascular  irregularity 
is  present  in  a  particular  case. 


i5° 


THE  PHYSIOLOGY  OF  TWINNING 


a)  slight 

b)  moderate  >  asymmetry  in  the  third  circulation 

c)  great        J  . 

,.    .  ,  .  ,  .,.     ,  I  and  simultaneous 

d)  interruption  of  current  in  umbiUca  artery  asymmetry  of 

e)  interruption  of  current  in  umbilical  vein    |  the  third  circulation 

/)    primary  heart-death  in  one  twin  (with  pronounced  asymmetry 

of  third  circulation) 
g)  destruction  of  one-half  of  the  placenta 

The  effects  upon  the  favored  twin  (the  one  that  acquires 
an  excess  of  blood)  differ  in  every  case  from  those  upon 
the  injured  twin  (the  one  that  suffers  a  diminution  of 
blood).     In  all  there  would  appear  to  be  fifty-six  per- 
mutations and  combinations  of  the  possible  variables  in 
physiological  interrelations  between  one-egg  twins.     In 
nearly  half  of  the  combinations  either  the  favored,  or 
rarely  the  injured,  twin  shows  no  noticeable  departure 
from  the  normal;  but  it  seems  to  be  quite  probable  that 
many   of    these   apparently   normal   individuals   suffer 
physiologically  so  as  to  acquire  certain  functional  heart 
weaknesses  or  disorders,  and  it  may  well  be  that  the  very 
common  difference  in  vigor  or  vivacity  between  one-egg 
twins  is  the  result  of  an  intra-uterine  injury  of  the  same 
kind  but  of  lesser  degree  than  those  that  are  clearly 
recognized.     The  most  serious  results  are  found  in  con- 
nection with  placenta-type  D  and  the  vascular  condi- 
tions c,  d,  e  for  both  twins,  and  /  and  g  for  the  injured 
twin.     Moderately  serious  results  appear  in  connection 
with  placenta-type  A,  and  conditions  b,  c,  d,  e,  for  the 
favored  twin,  and  conditions  c,  d,  e,  for  the  injured  one; 
and  in  placenta- type  B,  and  conditions  b,  c,  d,  e,  for  the 
favored  twin,  and  c,  d,  e  for  the  injured  one.     So  few 
examples  of  placenta-type  C  occur  that  the  situation  is 
less  clear  and  may  be  omitted  from  the  present  discussion. 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWINS  i  =;i 

Complete    acardia    (holoacardia)     occurs    only    in 

placenta-type  D  (in  which  in  addition  to  villous  trai 
fusion,  there  are  both  arterial  and  venous  anastromo 
in  connection  with  vascular  conditions  <i  and  e  for  the 
favored  twin  and  d,c,f,  and  g  for  the  injured  twin.     Hemi 
acardia  occurs  only  in  placenta-type  I)  under  exactly 
the  same  conditions  as  does  holoacardia.     Various  de- 
grees of  macrocardia  and  microcardia  occur  frequently  in 
connection  with  all  four  placenta- types  and  in  conn< 
tion  with  most  of  the  vascular  conditions.     When  one 
twin    of    a    pair    shows   microcardia    the    other    shows 
macrocardia.     In  Figure  50  is  shown  a  typical  placenta 
and  the  two  hearts  of  the  associated  twins,  that  on  the 
left  being  microcardiac  and  that  on  the  right  macro- 
cardiac.     Undoubtedly  various  minor  degrees  of  micro- 
cardia and  macrocardia  exist  in  even  supposedly  normal 
one-egg  twins,  and  these  conditions  may  have  a  perma- 
nent effect  on  their  vitality. 

MORPHOLOGICAL   RESULTS   OF   ACARDIA 

Naturally  we  should  expect  that  acardia  would  be 
accompanied  by  other  more  or  less  serious  consequena 
for  no  pronounced  slowing  up  or  deficiency  of  Mood 
could  occur  without  affecting  the  developing  fetus.  In 
general  it  may  be  said  that  the  particular  defects  in- 
duced depend  more  or  less  directly  upon  the  magni- 
tude of  the  departure  from  normal  and  the  particular 
type  of  vascular  disturbance  which  has  brought  about  the 
acardia.  Schatz  distinguishes  the  following  well-defined 
types  of  acardii  on  the  basis  of  their  general  morphology  : 

I.  Acardii  completi,  which  possess  head   and   trunk. 
but  may  be  more  or  less  deficient  in  amis  <>r  K 


152 


THE  PHYSIOLOGY  OF  TWINNING 


2.  Acardii  acormi,  which  possess  only  the  head  and 
the  merest  rudiments  of  body  and  extremities  (Fig.  52). 


FlG  5  2  — A  bizarre  type  of  heartless  human  one-egg  twin,  which  had 
a  nearly  normal  partner,  born  alive  (see  severed  placenta  of  the  latter 
turned  to  the  left).  This  monster  is  technically  an  acardius  acormus 
(heartless,  trunkless).  It  is  little  more  than  a  large  head  attached 
to  the  placenta  by  an  umbilical  cord.  Such  a  monster  is  kept  alive  by 
borrowing  blood  from  its  more  fortunate  twin  partner.     (After  Schatz.) 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWIN 

3.  Acardii  acephali,  which  possess  a  body  and  at  I<- 
some  of  the  extremities  fairly  well  developed,  but   qo 

head  (Fig.  53). 


Fig.  53. — A  pair  of  one-egg  human  twin  fetuses  attached  to  the  com- 
mon placenta.     The  one  on  the  left  hand  is  headless  and  has  ;i  dead 
heart.     It  is  an  example  of  the  type  of  twin  known  as  acardius  acephat 
(After  Schatz.) 

4.  Acardii  amor  phi  in  which  the  whole  organism  is 
a  shapeless,  rounded  mass  with  only  traces  of  head  and 
appendages  (Fig.  54,  p.  154). 

There  are  various  more  or  less  well  defined  types  oi 
acardii  amorphi,  distinguished  by  various  vascular 
peculiarities. 

One  of  each  of  these  types  of  acardii  is  shown  in  the 
accompanying  figures.     Some   of   them,   especially    the 


154 


THE  PHYSIOLOGY  OF  TWINNING 


Fig.  54. — A  very  abnormal  human 
twin  fetus  which  has  resulted  from  early 
heart-death  and  has  developed  badly 
owing  to  the  paucity  of  the  borrowed 
blood  supply  from  the  "favored"  twin. 
Such  a  twin  is  practically  unrecognizable 
as  a  human  creature  and  is  an  acardius 
amorphus.     (After  Schatz.) 


acormi,  are,  perhaps, 
the  most  bizarre  ex- 
amples of  human 
teratoma.  Schatz  de- 
scribes considerable 
numbers  of  such  mon- 
strous types  in  great 
detail  and  in  each  case 
suggests,  on  the  basis 
of  a  study  of  body  and 
placental  vascular 
conditions,  the  prob- 
able cause  of  each. 
These  are  matters  too 
far  removed  from  our 
present  interest  to  war- 
rant discussion  here. 

Up  till  now  we  have 
confined  our  attention 
to  conditions  in  en- 
tirely separate  human 
one-egg  twins.    It  now 


remains  for  us  to  make  a  brief  survey  of  conditions  in 
human  double  monsters  or  conjoined  twins. 


CONJOINED   TWINS 

ON   THE   INFLUENCES   WHICH   CONJOINED   TWINS 
EXERT   UPON   EACH   OTHER 

Just  as  in  separate  human  twins  one  individual  may 
be  smaller  in  size  or  defective  in  various  ways,  so  in 
conjoined  twins  the  component  individuals  may  be  very 
unequal  in  size  and  normality  of  structure. 


DEVELOPMENTAL  HAZARDS  OF  HUMAN  TWINS    I 

In  cases  of  marked  difference  between  the  two  com- 
ponents the  larger  more  nearly  normal  individual  li 
come  to  be  called  the  autosite,  and  the  -mailer  more  or 
less  abnormal  individual,  the  parasite.     This  terminol 
has  probably  certain  false  implications.     It  implies  either 
that  one  of  the  individuals  is  in  some  sense  an  origi 
nally  superior  or  primary  one  and  that  the  other  is  a  sort 
of  secondary  outgrowth  of  the  first  produced  by  budding, 
or  that  the  parasite  has  come  to  attach  itself  secondarily 
by  fusion  to  the  body  of  the  primary  individual.     Both 
of  these  alternative  implications  are,  I  believe,  obviously 
ill   founded,    as  will  be  appreciated   when    it   becomes 
known  that  the  same  vascular  anastomoses  exist  between 
conjoined  twins  as  prevail  for  separate  one-egg  twins. 

CONJOINED   TWINS  WITH   SEPARATE   HEARTS    AND 
SEPARATE   UMBILICAL    CORDS 

The  majority  of  conjoined  twins  are  joined  only  by 
the  body  wall  and  have  most  of  the  viscera  separate. 
There  are  therefore  the  same  opportunities  tor  one 
component  to  injure  the  other  through  interference  with 
its  blood  supply  as  if  they  were  not  united  at  all.  Schatz 
cites  a  large  number  of  instances  of  acardia,  mediacardia, 
microcardia,  and  macrocardia  in  conjoined  twins,  and 
makes  scarcely  any  distinction  on  the  grounds  oi  sep- 
arateness  or  union  of  twins.  He  seems  to  take  it  for 
granted  that  the  two  types  are  simply  two  expressions 
of  the  same  phenomenon.  Without  going  into  any 
great  detail  then,  it  would  appear  that  the  developmental 
hazards  of  conjoined  twins  are  the  same  as  those  oi 
separate  twins  and  we  may  expect  to  find  the  same  kinds 
of    inhibited    individuals.     In    I  where    compl 


156  THE  PHYSIOLOGY  OF  TWINNING 

acardia  develops  we  may  expect  the  injured  component 
of  a  pair  of  conjoined  twins  to  exhibit  the  most  pro- 
found deterioration,  equivalent  to  acormi,  acephali,  and 
amorphi.  Thus  the  injured  individual  may  be  reduced 
to  merely  an  extra  abnormal  head,  an  additional  limb 
or  pair  of  limbs,  or  to  a  shapeless  mass  of  more  or 
less  differentiated  tissues  surrounded  by  a  cyst.  The 
opportunity  for  complete  suppression  of  the  injured 
twin  is  greater  in  the  case  of  conjoined  than  in  that 
of  separate  twins,  because  of  the  fact  that  the  two  are 
in  such  close  contact.  This  relation  makes  it  possible 
for  the  tissues  of  the  stronger  component  to  grow  more 
or  less  completely  around  the  weaker  component  and 
to  inclose  it.  Thus  we  may  have  a  certain  amount  of 
regulatory  growth  tending  to  obliterate  the  effects  of 
incomplete  twinning.  Cases  have  not  infrequently  been 
observed  in  which  an  individual  apparently  quite  nor- 
mal has  had  removed  from  the  abdominal  cavity  a 
tumor  which,  on  examination,  has  turned  out  to  be 
the  amorphous  remains  of  a  formerly  conjoined  twin. 
Such  a  regulation  from  conjoined  twinning  back  to  a 
nearly  normal  single  condition  has  been  observed  in  twin 
starfish  larvae  and  has  already  been  discussed  (pp.  26,  27) . 

CONJOINED   TWINS   WITH   SINGLE    UMBILICAL   CORDS 
BUT   SEPARATE   HEARTS 

There  seem  to  be  very  few  cases  of  conjoined  twins 
that  can  be  interpreted  as  having  originally  had  only 
one  heart.  If,  in  advanced  stages,  the  autosite  has  a 
heart  and  the  parasite  none  we  must  conclude  that  the 
latter  is  an  acardius  which  has  secondarily  lost  its  heart. 
Where  there  is  only  one  umbilical  cord  and  only  one  set 


DEVELOPMENTAL  HAZARDS  01    III  MAN  TWINS     i 

of  umbilical  vessels  it  is  obvious  that  we  cannot  mt 

for  the  inequality  of  the  twin  component-  as  the  result 
of  asymmetry  in  the  third  circulation  of  the  placenta. 
Such  a  condition  could  only  be  due  to  some  asymmetry 
or  inequality  in  the  bodies  of  the  components  them- 
selves. Schatz  considers  that  one  of  the  chief  soun 
of  initial  inequality  is  partial  situs  inversus  viscerum. 
He  has  observed  in  double  monsters  not  a  few  cases  in 
which  a  partial  situs  inversus  of  the  heart  or  main  blood 
vessels  causes  unfavorable  circulatory  relations.  In 
complete  situs  inversus  the  components  would  doubtli 
be  equally  favorably  related  to  their  respective  body 
parts,  but  a  slight  degree  or  any  incomplete  degree  of 
situs  inversus  will  be  unfavorable.  Any  initial  handi- 
cap, when  there  is  competition  for  a  single  blood  supply, 
wrould  doubtless  result  in  progressive  gain  of  advanta 
on  the  part  of  the  individual  initially  favored  and  a 
progressive  loss  of  ground  by  the  less  fortunate  twin. 
Thus  might  arise  the  conditions  of  autositism  and  para- 
sitism. 

In  a  previous  connection  (p.  67),  when  discussing 
the  causes  of  inequality  in  the  components  of  double 
monsters  in  fishes,  we  offered  as  one  explanation  oi 
this  inequality  that  there  is  probably  as  much  vascular 
anastomosis  on  the  vitellum  of  the  fish  as  on  the  common 
placenta  of  human  twins.  Now  that  the  reader  h 
seen  how  great  an  interinfluence  actually  does  exist 
between  the  components  of  conjoined  twins  and  that 
the  mechanism  of  this  interfluence  is  largely  one  involv- 
ing unequal  blood  transfusion  between  the  components, 
he  will  appreciate  the  force  oJ  the  argument  against 
Stockard's  interpretation  of  the  cause  of  the  different 


158  THE  PHYSIOLOGY  OF  TWINNING 

in  size  and  degree  of  normality  of  the  components  of 
fish  double  monsters.  In  conclusion  I  would  also  like 
further  to  urge  another  explanation  of  the  inequality  of 
the  two  components  that  was  previously  offered  in  the 
case  of  fishes  (see  pp.  67,  68) :  that  the  two  bilateral  pri- 
mordia,  after  physiological  isolation,  are  somewhat  inde- 
pendent and  are  not  affected  to  the  same  extent  by  the 
prevailing  growth-retarding  agencies  that  have  been 
responsible  for  twinning.  There  is  just  as  much  reason 
for  the  two  bilateral  components  of  a  pair  of  conjoined 
twins  to  be  different  in  their  susceptibility  to  inhibiting 
agents  as  there  is  for  the  two  sides  of  a  single  individual 
to  be  differently  affected.  The  same  causes  are  almost 
certainly  concerned  in  the  same  ways  in  both  cases. 


CHAPTKk  XI 

HEMIHYPERTROPHY     A  TYPE  OF  MINIMAL 
TWINNING  IN  MAN 

Hemihypertrophy  is  a  relatively  rare  anomaly   <>l> 
served   in   human  beings,   which  has  been    defined 
"an  overgrowth  of  one  half  or  one  side  of  the  body  or  of 
a  part." 

Quite  recently  Dr.  Arnold  Gesell  has  presented  an 
interesting  and  illuminating  discussion  of  the  etiology  of 
this  condition  which  seems  to  lead  to  the  conclusion 
that  it  is,  in  all  probability,  one  expression  of  the  wide- 
spread phenomenon  of  bilateral  twinning. 

There  is  always  a  certain  amount  of  asymmetry 
between  the  right-  and  left-hand  sides  of  individuals  or 
between  the  paired  bilateral  structures.  No  human 
being  is  exactly  the  same  on  the  two  sides,  but  usually 
the  discrepancy  in  the  two  sides  is  not  sufficient  to  excite 
comment.  Occasionally,  however,  individuals  have  come 
to  the  attention  of  physicians  and  psychiatrists  in  which 
the  asymmetry  is  very  striking.  In  the  paper  of  Dr. 
Gesell  (1921)  mentioned  above  a  detailed  account  i- 
given  of  a  youth,  twenty  years  of  age,  who  was  very 
strikingly  larger  in  all  the  organs  of  the  right  side,  head. 
face,  anus,  legs,  trunk.  Even  such  median  structures  as 
the  nose  and  the  penis  were  distinctly  larger  ^n  the  riurht 
side  than  on  the  left.  Mentally  this  boy  "must  he 
classified  as  an  imbecile/5  "The  fa<  ts  clearly  indicate,' 
says  the  author,  "  that  hemihypertrophy  should  he  added 

159 


160  THE  PHYSIOLOGY  OF  TWINNING 

to  the  list  of  developmental  anomalies  which  bear  some 
lawful  relation  to  the  incidence  of  mental  deficiency." 

The  author  tabulates  forty  cases  of  total  hemihyper- 
trophy  that  have  been  reported  in  the  literature, 
and  twenty-three  cases  of  partial  hemihypertrophy,  a 
considerable  proportion  of  which  are  associated  with 
mental  defectiveness.  In  discussing  the  relation  of 
hemihypertrophy  to  mental  defects  he  says: 

The  frequent  association  of  hemihypertrophy  and  of  cranial 
asymmetry  with  mental  defect  and  the  consistent  preference  in 
both  conditions  for  enlargement  of  the  right  side  suggests  some 
lawful  grouping  of  causative  factors.  It  is  quite  possible  that 
the  origin  of  certain  altogether  obscure  cases  of  secondary 
amentia  may  lie  in  an  undetectable  but  decisive  imbalance  of  the 
fundamental  process  of  twinning  which  follows  fertilization. 

In  addition  to  the  prevalence  of  mental  defects  in 
connection  with  hemihypertrophy  there  occur  a  number 
of  other  abnormal  conditions  always  on  the  hyper- 
trophied  side.  Among  the  commonest  of  these  are 
various  skin  peculiarities  such  as  surface  dilation  of 
capillaries  causing  red  spots,  large  hairy  moles,  blue 
pigmentation,  dark-red  pigmentation,  excessive  hairi- 
ness. Some  other  peculiarities  of  the  hypertrophied 
side  are  also  of  interest.  The  hair  may  be  colored 
differently  on  the  two  sides,  that  of  the  hypertrophied 
side  being  longer  and  darker;  the  temperature  may  be 
as  much  as  two  to  four  degrees  higher;  the  epiglottis  shows 
reversed  asymmetry.  The  fact  that  about  70  per  cent 
of  the  cases  listed  involve  hypertrophy  of  the  right  side 
suggests  the  possibility  of  symmetry  reversal,  but  there 
is  no  data  to  show  that  there  ever  occurs  any  true 
situs  inversus  viscerum.    The  only  instance  of  the  sort 


HEMIHYPERTROPHY  16] 

is  that  given  above,  viz.,  that  there  was  noted  in  one  ca 
of  right-handed  hemihypertrophy  a  reversed  asymmetry 
of  the  epiglottis. 

An  interesting  discussion  entitled  "Twinning  and 
Asymmetry1'  in  Dr.  Gesell's  paper  appears  to  me  to  be 
of  sufficient  interest  to  deserve  quoting  at  some  Length: 

TWINNING  AND   ASYMMETRY 

It  is  natural  that  a  discussion  of  the  etiology  of  hemihyper- 
trophy should  finally  bring  us  to  problems  of  double  psychical 
personality  and  twins.  Further  researches  into  the  biology  of 
twinning  may  bring  the  remarkable  phenomenon  of  unilateral 
hypertrophy  more  completely  within  our  comprehension;  may 
even  prove  it  to  be  on  closer  scrutiny  more  frequent  and  l< 
anomalous  than  we  had  supposed.  Indeed,  even  now,  all  thi: 
considered,  the  real  marvel  is  not  the  occurrence  of  hypertrophy 
but  the  fact  that  hemihypertrophy  is  such  an  extreme  rarity. 

By  twinning  we  mean  the  production  of  equivalent  structures 
by  division.  This  statement  is  taken  from  the  biologist  Bateson, 
who  regards  the  power  to  divide  as  a  fundamental  attribute  of 
life.  The  tendency  to  symmetry,  to  bilateral  equivalence  or 
mirror-imaging  is  so  general  that  it  must  be  regarded  as  a  funda- 
mental of  biologic  mechanics.  Hemihypertrophy  accordingly  may 
be  conceived  as  some  profound  inaccuracy  in  the  natural  pr 
of  developmental  duplicity.  It  is  not  as  monstrous  as  the  double 
monsters,  but  it  may  have  a  related  morphogenesis.  At  any 
rate,  we  can  safely  assume  that  hemihypertrophy  is  not  an  artifa<  t 
really  consisting  in  a  hemi-atrophy.  It  is  evident!)  a  mild 
unilateral  gigantism  of  an  individual  whose  Lesser  somatic  halt  is 
normal. 

In  a  certain  biologic  sense  we  may  regard  every  bilateral 
individual  as  being  a  pair  of  twins.  II.  11.  Newman,  in  his  fascinat- 
ing work  on  The  Biology  of  Twins,  holds  that  mon  >l  ic  t  winni 
— where  a  single  egg  produces  two  offspring  -is  "a  phenomenon 
that  should  be  considered  as  only  a  phase  of  the  much  more  general 
phenomenon  of  symmetrical  division.      The  development  ol   the 


162  THE  PHYSIOLOGY  OF  TWINNING 

right-  and  left-hand  homologous  organs  in  a  bilateral  organism 
is  essentially  a  twinning  process."  This  author  also  observes  that 
"the  whole  matter  of  bilateral  development  appears  to  be  quanti- 
tative in  nature,  in  that  the  same  type  of  process  may  go  not  so 
far  or  farther  than  normal."  Just  as  there  may  be  an  inhibition 
of  the  normal  culmination  of  the  process  of  bilateral  division 
(as  in  the  median  cyclopic  eye),  so  there  is  frequently  an  excess 
of  division  resulting  in  two  bilateral  structures  becoming  com- 
pletely segregated,  as  when  a  single  individual  develops  two  heads 
or  two  tails,  while  the  remainder  of  the  organism  is  a  more  or 
less  normal  individual.  Newman,  like  Bateson,  regards  the  phe- 
nomenon of  twinning  as  a  fundamental  process  which  is  almost 
universal  in  the  field  of  biology. 

From  this  point  of  view,  hemihypertrophy  may  be  interpreted 
as  an  atypical  or  incomplete  form  of  twinning,  a  variant  of  the 
same  process  which  may  produce  a  double-headed  monster  or  a 
perfectly  ordinary  normal  individual — an  ordinary  individual 
being  an  organism  in  whom  there  has  been  a  precisely  balanced 
inhibition  on  the  biologic  process  of  bilateral  doubling. 

DISCUSSION 

If  we  accept  hemihypertrophy  as  a  mild  form  of 
unilateral  gigantism  we  must  view  the  hypertrophied 
side  as  abnormal  and  the  small  side  as  relatively  normal. 
From  this  point  of  view  we  are  led  to  compare  the 
condition  in  question  with  one  that  has  been  frequently 
noted  in  connection  with  various  unilateral  monstrosities 
in  fishes  which  are  the  result  of  certain  growth-retarding 
agents.  It  very  commonly  happens  in  such  fish  embryos 
or  larvae  that  the  organs  of  one  side  of  the  body  develop 
much  more  rapidly  than  those  of  the  other  side  and  we 
get  curved  or  even  spiral  forms.  Frequently  the  eye 
of  one  side  is  much  more  defective  than  that  of  the  other, 
and  the  same  is  true  of  fins  and  other  paired  structures. 
In  view  of  these  facts  we  might  interpret  these  cases, 


HEMIHYPERTROI'IIY  [63 

and  perhaps  also  cases  of  human  hemihypertrophy,  as 
due  to  a  physical  inequality  of  the  two  bilateral  primordia 
so  that  one  side  would  be  more  affected  by  subnormal 
conditions  than  the  other.  Even  in  cases  of  double 
monsters  there  is  often  a  pronounced  difference  in  tin- 
two  components  that  is  probably  based  upon  some  early 
physiological  inequality  of  the  two  bilateral  primordia. 
Even  with  this  possible  explanation  of  asymme- 
try in  mind  I  find  myself  in  essential  agreement  with 
Dr.  Gesell  in  his  interpretation  of  hemihypertrophy  as  a 
minimal  phase  of  double  monstrosity.  We  may  conchn  le 
that,  since  twinning  in  general  consists  of  a  more  <>r 
less  complete  isolation,  physiological  at  first  and  later 
physical,  of  the  bilateral  primordia  of  a  single  embryonic 
axis,  there  may  readily  occur  slight  degrees  of  physio- 
logical isolation  or  independence  of  the  two  halves  of  the 
body.  Hemihypertrophy,  complete  and  incomplete. 
would  then  be  the  result  of  such  relatively  slight  isolation 
and  would  therefore  logically  belong  in  the  same  series 
with  duplicate  twins,  double  monsters,  and  other  typ 
of  twins.  The  unilateral  hypertrophy  would  be  thought 
of  as  the  result  of  some  deficiency  confined  t»>  one  side, 
and  the  associated  peculiarities  would  be  viewed  as 
secondary  consequences  of  the  primary  deficiency. 


CHAPTER  XII 

SYMMETRY  REVERSAL  AND  MIRROR-IMAGING  IN 
TWINS  AND  DOUBLE  MONSTERS 

SYMMETRY  REVERSAL   IN  HUMAN   TWINS 

Ever  since  double  monsters  began  to  attract  the  atten- 
tion of  biologists  the  occurrence  of  reversed  symmetry 
has  been  a  matter  of  marked  interest.     The  earlier  obser- 
vations are  confined  to  instances  of  reversal  of  the  asym- 
metry of  certain  unpaired  organs  such  as  stomach,  heart, 
aortic  arch,  vena  cava.     The  familiar  condition  in  which 
the  greater  curvature  of  the  stomach  is  to  the  left,  the 
apex  of  the  heart  to  the  left,  aortic  arch  to  the  left,  and 
vena  cava  to  the  right  is  by  the  older  writers   called 
situs  solitus— the  situation  normal  for  single  individuals. 
The  unusual  condition  in  which  the  greater  curvature 
of  the  stomach  is  to  the  right,  apex  of  heart  to  the  right, 
aorta  to  the  right,  and  vena  cava  to  the  left  was  at  first 
called  situs  viscerum  transversus.     The  older  authors  also 
use  for  this  phenomenon  the  terms  situs  rarior,  solito 
inversus,  and  situs  inversus  viscerum.     Among  English- 
speaking  writers  it  has  come  to  be  referred  to  as  symmetry 
reversal  or  mirror-imaging.     Rare  instances  appeared  in 
which    a    single-born    individual    exhibited    symmetry 
reversal,  and  it  has  commonly  been  suggested  that  such 
individuals    are    probably    surviving    right-hand    com- 
ponents of  twin  pairs  in  which  the  left-hand  component 
was  either  formerly  a  parasite,   afterward  completely 
resorbed,  or  else  had  died  at  an  early  stage. 

164 


SYMMETRY  REVERSAL  AND  MIRkOk-IM  \(il\<;   165 

In  human  conjoined  twins,  especially  in  those  known 
as  anadidymi,  in  which  there  arc  two  separate  heads 
while  the  bodies  are  united  in  the  thoracic  and  abdominal 
regions,  symmetry  reversal  is  quite  common.  A<  1  ording 
to  Spaeth  (i860)  there  are  all  degrees  of  situs  inversus 
viscerum  ranging  from  cases  of  complete  reversal  to  1 
in  which  there  is  only  a  trace  of  it.  Many  cases  of  slight 
symmetry  reversal  are  overlooked,  but  it  is  these  marginal 
cases,  which,  according  to  Schatz  (1887),  involve  tin- 
greatest  danger  for  the  twins  possessing  them.  They  are 
nearly  always  associated  with  some  irregularity  in  the 
circulatory  system  and  such  irregularities  are  almost 
certain  to  have  serious  consequences  when  the  blood 
supplies  of  the  two  individuals  come  to  be  united  by 
placental  anastomoses.  Not  only  are  there  numerous 
instances  of  situs  inversus  in  conjoined  twins  but,  accord- 
ing to  Spaeth  and  Schatz,  small  deviations  from  the 
normal  situs  are  often  found  in  separate  one-egg  twins. 
Such  deviations  do  not  so  much  concern  the  larger 
more  obvious  organs,  such  as  stomach,  heart,  aorta,  but 
have  to  do  especially  with  less  conspicuous  regions  of 
the  circulatory  system. 

Schatz  in  studying  separate  human  one-egg  twins, 
especially  the  acardii  dealt  with  in  chapter  \.  noted 
many  instances  of  incomplete  situs  inversus 
In  one  case  the  only  reversal  present  had  to  do  with  the 
chief  vessels  of  the  heart;  the  aorta  was  on  the  right, 
the  vena  cava  on  the  left.  In  another  case  the  aorta 
was  to  the  left  of  the  vertebral  column  yet  to  the  right 
of  the  vena  cava.  Another  case  was  noted  in  which 
there  was  practically  bilateral  symmetry  of  certain 
normally   asymmetrical   structure-.      I     en    these    les 


i66 


THE  PHYSIOLOGY  OF  TWINNING 


departures  from  the  normal  situs  solitus  are  to  be  inter- 
preted as  evidences  of  incomplete  symmetry  reversal. 
Complete  situs  inversus  in  separate  one-egg  twins  is 
rare;  but  one  conspicuous  instance  has  come  to  my 
attention.  This  is  the  case  of  Kuchenmeister  (1883) 
who  describes  a  pair  of  nearly  full-term  twins  one  of 

which  had  complete  situs 
solitus,  the  other  complete 
situs  inversus. 

In  this  place  it  is  not  my 
purpose  to  present  any  large 
amount  of  data  on  situs 
inversus  viscerum  in  human 
twins.  Morrill  (191 9)  has 
recently  given  us  a  concise 
survey  of  the  literature  on 
this  subject.  He  himself  has 
made  a  detailed  study  of  a 
double-headed  monster  that 
came  to  his  attention  and 
has  figured  the  monster  in 
its  entirety  (Fig.  55)  as  well 
as  its  visceral  conditions 
(Fig.  56).  This  is  obviously 
a  case  of  complete  situs  in- 
versus viscerum  and  is  of  very  great  interest.  Morrill 
calls  attention  to  the  existence  of  situs  inversus  in  the 
cases  of  several  rather  famous  diplopagi.  Eichwald 
states  "that  the  thoracopagous  monsters  examined  by 
him  showed,  in  almost  every  case,  some  transportation 
of  the  viscera  in  one  of  the  bodies,  though  to  a  vary- 
ing extent."     The  pygopagous  Carolina  twins,  Millie- 


FiG.  55. — A  striking  example 
of  a  human  double  monster 
(anadidymus)  which  had  per- 
fect situs  inversus  viscerum. 
(After  a  photograph  published 
by  Morrill.) 


SYMMETRY  REVERSAL  AND  MIRROR-IMAGING   167 


Christina  (colored),  are  declared  to  have  had  situs 
inversus  of  at  least  the  heart.  One  of  the  famous 
Siamese  Twins  is  stated  to  have  had  a  partial  reversal 
of  viscera. 

A    few  significant  conclusions  may  be  stated  in  r< 
viewing  the  matter  of  symmetry  reversal  in  human  twins: 

a)  The  con- 
dition is  obvi- 
ously associated 
with  a  form  of 
twinning  in 
which  the  two 
individuals  are 
derived  from  a 
primord  i  um 
that  would  nor- 
mally produce 
the  right-  and 
left-hand  halves 
of  a  bilateral  in- 
dividual. 

b)  The  left- 
hand  individual 
always  retains 
the  normal  situs 
and  the  right-hand  individual  very  commonly  sho 
some  degree  of  symmetry  reversal. 

c)  It  is  very  rare  indeed  thai  separate  human  twin- 
show  any  signs  of  situs  inversus  viscerumy  but   they  do 
not  uncommonly  show  mirror-imaging  in  certain  minor 
external  characters  such  as  finger-prints   see  Tk  Biolc 
of  Twins,  pp.  159-60). 


Fig.  56. — The  viscera  of  the  double  monster 
shown  in  Viis,.  ^^,  laid  out  in  somewhat  schematic 
fashion  to  show  the  complete  situ 
cerum.     (From  .Morrill.) 


1 68 


THE  PHYSIOLOGY  OF  TWINNING 


d)  If  we  include  minor  degrees  of  symmetry  reversal 
we  find  it  to  be  a  much  more  frequently  occurring 
phenomenon  than  we  had  formerly  supposed.  Complete 
situs  solitus  in  right-hand  components  of  conjoined 
twins  is,  in  fact,  relatively  rare. 

e)  The  existence  of 
mirror-image  symmetry 
in  the  components  of 
double  monsters  argues 
against  the  theory  that 
they  have  arisen  through 
the  fusion  of  separate 
embryonic  axes  and  for 
the  theory  of  origin 
through  dichotomy  or 
fission  of  the  bilateral 
halves  of  a  single  em- 
bryonic axis. 

SYMMETRY  REVERSAL  IN 
FISH  TWINS 

Until  very   recently 
no    attention   seems    to 
have  been  paid  to  sym- 
metry   reversal   in   fish 
twins.     Gemmill  (1912), 
although   he   figures  in 
great  detail  numerous  conditions  that  are   essentially 
symmetry  reversals,  never  calls  attention  to  the  con- 
dition as  such.     One  cannot  help  but  note  the  beauti- 
fully perfect  mirror-imaging  displayed  in  his  horizontal 
sections   through  the  heads  of  partially  double-headed 


Fig.  57.  —  A  horizontal  section 
through  the  head  region  of  a  partially 
double  trout  embryo.  The  embryo  is 
double  as  far  back  as  the  anterior  end 
of  the  mid-brain.  Note  the  perfect 
mirror-imaging  of  the  twinned  struc- 
tures.    (After  Gemmill.) 


SYMMETRY  REVERSAL  AND  MIRROR-IMAGING    1 


monsters  (Fig.  57).     Each  side  is  a   practically 

mirror-image  of  the  other,  the  inner  side-  in  each  partly 
divided  head  being  less  fully  developed  than  the  outer. 
We  note  another  equally  beautiful  instance  of  mirror 
imaging  in   the  skulls  of  double-headed  monsters.     In 
Figure  58  is  shown  such 
a  skull  in  which  all  outer 
bones    in   both   compo- 
nents are  perfect  and  a 
number  of  inner  bones 
imperfectly     duplicated 
or  single. 

Certain  beautiful 
cases  of  duplicity  of 
visceral  parts  are  also 
figured,  such  as  heart  and 
the  urogenital  system. 
Especially  interesting 
are  the  conditions  in 
some  ol  these  hearts  in 


which    various    degrees 


Fig.  58. — -The  skull  and  jaw  of  a 
partially  double-headed  young  trout, 
showing    perfect    mirror-imaging 
twinned  structures.    (After  Gemmill.) 

of  duplicity  are  shown. 

Figure  59  (p.  170)  shows  the  heart  of  a  normal  fish  embryo 
before  it  has  assumed  any  marked  degree  of  asymmetry. 
Figure  60  (p.  170)  shows  a  case  of  partial  duplicity,  es] 
cially  complete  at  the  ventricular  em  1.     Figure6i    p.17 
is  an  example  of  almost  complete  duplicity  of  the  heart, 
only  the  inner  vessels  being  united,  especially  the  inner- 
vena  cava.    The  two  halves  are  perfect  mirror  imagi 
One  step  farther  and   the  two  hearts  with  their  main 
vessels  would  be  entirely  separated  the  one  set  from  the 
other.    Were  such  a  further  separation  to  occur  we  would 


170 


THE  PHYSIOLOGY  OF  TWINNING 


expect  a  continuance  of  the  mirror-imaging.  Gemmill 
does  not  give  us  any  further  data  on  this  point,  but  two 
recent  studies  on  inverse  symmetry  in  double-monster 
trout  have  supplied  this  deficiency. 

Morrill  (1919)  has  worked  on  the  internal  anatomy 
of  some  of  Stockard's  collection  of  double  trout.     He 


Figs.  59-61. — Three  diagrams  showing  symmetry  reversal  in  the 
hearts  and  principal  vessels  of  trout  double  monsters.  Fig.  59  shows 
the  normal  single  heart.  Fig.  60  shows  an  incompletely  twinned  heart. 
Fig.  61  shows  completely  divided  twinned  hearts,  the  right  component  of 
which  shows  situs  inversus  viscerum.     (After  Gemmill.) 

found  several  cases  of  complete  symmetry  reversal  and 
also  several  cases  that  were  incomplete  or  doubtful. 
Never  was  there  any  symmetry  reversal  in  the  indi- 
viduals in  which  body  and  tail  were  both  separate,  but 
only  in  cases  in  which  heads  were  entirely  separate  and 
the  viscera  separate  at  least  back  to  the  pelvic  region. 


SYMMETRY  REVERSAL  AND  MIRROR  IMAGING   171 

In  the  cases  of  typical  situs  inversus  the  left  hand  com- 
ponent  shows   the   normal    situs   and    the    right  hand 

component  the  reversed  situs  of  all  asymmetrical  struc- 
tures. Morrill  states,  however,  that  mirror-imaging, 
even  in  the  types  of  duplicity  described  as  most  favorable 
for  its  appearance,  is  by  no  means  the  rule,  but  that  the 
majority  of  specimens  show  the  normal  situs  in  both 
components.  On  the  basis  of  this  finding  he  concludes 
that  reversal  of  symmetry  is  rare  in  fishes 

Somewhat  more  recently  Swett  (1921)  has  studied 
the  subject  in  another  collection  of  somewhat  more 
advanced  trout  embryos.  He  also  finds  that  situs 
inversus  is  confined  to  monsters  showing  a  certain 
limited  range  of  duplicity.     Swett  says: 

It  will  be  seen  that  those  whose  vertebral  columns  arc  fused 
anterior  to  the  dorsal  fin  and  those  that  arc  nearly  separate  sh< 
at  best  only  doubtful  mirror-imaging.     It   is   further  noted, 
was  also  found  by  Morrill,  that  not  all  the  animals  falling  within 
the  limits  of  doubling  apparently  most  favorable  for  transposition 
of  the  viscera  show  this  phenomenon.      Thus  there  are  indications 
that  the  reversal  of  asymmetry  is  not  a  necessary  consequei 
any  condition  of  doubling,  but  may  or  may  not  occur,  depending 
on  some  other  factor  capable  of  operating  within  these  limits. 

Certain  additional  facts  appear  in  Swett's  paper. 
He  finds  that  mirror-imaging  of  the  digestive  tract  exists 
only  when  the  point  of  union  of  the  twin  tracts  occurs  at 
some  point  between  the  pylorus  and  the  point  ol  exit 
of  the  intestine  from  the  abdominal  cavity.  Another 
point  of  considerable  interest,  in  view  of  the  Lnterpreta 
tion  given  by  Stockard  of  the  nature  and  relationship  oi 
autosite  and  parasite  in  double  monster  fish,  is  thai  in 
Swett's  material  there  is  a  case  of  partial  situs 
in  the  parasite  (Fig.  62,  p.  172).     'Ibis  finding  far 


172  THE  PHYSIOLOGY  OF  TWINNING 

toward  corroborating  my  theory  that  even  two  unequal 
components  arise  through  the  separation  of  the  bilateral 
primordia  of  a  single  embryonic  axis,  and  that  one  half 
must  have  been  more  susceptible  to  retarding  agents  than 
the  other.  A  very  clear  case  of  typical  and  complete 
situs  inversus  is  shown  in  Figure  63  (p .  1 73) .  Several  other 
good  cases  are  figured  by  Swett.  It  may  also  be  said  that, 
even  in  a  much  smaller  collection  of  double  monsters, 


SB  L, 
Fig.  62. — Ventral  view  of  trout  double  monster  of  the  autosite- 
parasite  type,  showing  partial  situs  inversus  viscerum.  The  lobe  at  the 
left  of  the  figure  just  below  the  normal  head  is  the  reduced  parasite  com- 
ponent representing  the  inferior  or  right-hand  twin.  H,  heart;  L,  liver; 
SB,  swim  bladder;  S,  stomach;  7,  intestine.     (From  Swett.) 

Swett  found  more  cases  of  situs  inversus  than  did  Morrill 
in  a  larger  collection.  The  difference,  I  think,  is  largely 
one  of  interpretation.  Many  slight  degrees  of  reversal 
are  likely  to  be  overlooked.  In  all  probability  the  cases 
described  by  Morrill  as  uncertain  or  irregular  should  be 
diagnosed  as  examples  of  incomplete  situs  inversus. 

In  attempting  to  interpret  the  data  on  situs  inversus 
in  fishes  two  particular  conditions  need  to  be  emphasized : 

1.  There  is  no  reason  to  expect  to  find  situs  inversus 
except  in  double  individuals  derived  from  the  antimeric 
halves   of    a  single    embryonic   axis.     In   the   case  of 


SYMMETRY  REVERSAL  AND  MIRROR  [MAGING   I 

embryos  derived  from  two  embryonic  shields         b  of 
which  has  its  own  right-  and  left-hand  sides,   m  no 

reason  in  the  world  why  there  should  be  a  reversal  of 
the  viscera.     Each  is  from   the  beginning  a   separate 


Fig.  63. — A  ventrolateral  view  of  a  trout  double  monster  (anadidy- 
mus)  showing  situs  inversus  viscerum.     The  component  on  the  readi 
right  is  the  true  left-hand  component  and  has  the  typical   left  hand 
asymmetry  of  the  species.     The  component  on  the  reader's  lefl    [the 
right-hand  component),  has    the  reversed  asymmetry   of   the    \ 
Letters  have  the  same  significance  as  in  Fig.  62.     (From  Swetl 

individual  except  that  it  happens  to  be  on  the  same 
yolk  sac  as  its  twin  partner.  It  would  also  be  impossible 
to  say  which  of  such  twins  is  the  right-hand  and  which 
the  left-hand  component.  Therefore  right  handed  or 
reversed  asymmetry  would  be  quite  inappropriate   in 


174  THE  PHYSIOLOGY  OF  TWINNING 

either  one  of  them.  If  we  were  ever  to  find  situs  inversus 
in  twins  separate  except  for  the  common  yolk  sac,  it  is 
certain  that  the  explanation  of  such  an  occurrence 
would  offer  a  problem  of  extreme  difficulty.  Normal 
situs  in  both  of  these  twins,  which  I  have  classed  as 
separate  one-egg  twins,  is,  then,  only  to  be  expected. 
The  same  statement  may  be  made  for  those  double 
monsters  that  are  united  only  by  lateral  or  ventral  parts. 
Such  individuals  may  be  interpreted  as  resulting  from 
secondary  or  mechanical  fusions  due  to  the  close  proxim- 
ity of  two  embryonic  shields.  Unless  double  monsters 
have  a  considerable  single  region  which  is  like  that  of 
a  normal  fish,  I  would  say  that  such  forms  do  not  come 
from  the  antimeric  halves  of  a  single  axis  and  there- 
fore would  not  be  expected  to  show  situs  inversus. 

2.  In  the  well-defined  cases  of  twins  with  rather 
extensive  axial  regions  in  common,  such  as  the  types 
in  which  situs  inversus  occurs,  our  problem  is  not  so 
much  to  account  for  situs  inversus  (for  this  is  the  expected 
condition  in  such  bilaterally  related  twins)  as  to  account 
for  the  not  infrequent  occurrence  of  left-hand  asymme- 
try (situs  solitus)  in  the  right-hand  component.  This 
seems  to  be  the  real  symmetry  reversal,  for  the  right-hand 
component  is  expected  to  be  a  mirror-image  of  the  left, 
and  should  show  right-hand  asymmetry  (situs  inversus) 
of  asymmetrical  structures.  The  process  is  probably 
one  of  regulation.  The  right-hand  component,  which 
in  some  cases  retains  the  symmetry  of  the  half- 
primordium  from  which  it  came,  may  become  sufficiently 
independent  to  develop  its  own  asymmetry  just  as 
though  it  were  a  single  individual  in  no  way  influenced 
by    its    twin    component.     Abundant    instances    were 


SYMMETRY  REVERSAL  A XI )  M I R ROR-IMAGING    [75 

noted  by  the  writer  in  the  case  of  armadillo  quadruplets 
in  which  both  twins  showed  the  same  unilateral  asym- 
metry in  their  scute  peculiarities.  It  was  about  equally 
common,  however,  for  them  to  show  mirror-imaj  m- 
metry.  Here  we  have  a  somewhat  parallel  case  and 
the  explanation  is  the  same:  that  there  is  in  smiir  cas 
a  greater  degree  of  isolation  between  the  twin  components 
than  in  others,  so  that  sometimes  the  two  individuals 
act  as  though  entirely  independent,  and  in  others  as 
though  they  still  retained  some  residuum  of  the  earlier 
interrelationship  characteristic  of  antimeric  halves  of  a 
single  individual,  a  relation  that  expresses  itself  m< 
obviously  in  mirror-imaging. 

In  concluding  this  discussion  of  mirror-ima^in^  in 
fish  monsters  I  would  like  to  reiterate  what  appears  t<» 
me  a  very  fundamental  principle:  that  mirror-imaging 
is  normal  for  twins  derived  through  separation  of  the 
antimeric  halves  of  a  single  embryonic  axis.  Such  indi- 
viduals rarely,  possibly  never,  become  entirely  separate. 
There  is  no  reason  to  expect  situs  inversus  in  separate 
twins  that  have  originated  from  two  complete  embryonic 
axes  on  a  single  blastoderm. 

EXPERIMENTAL  PRODUCTION  OF  SITUS  INVERSi 

VISCERUM    I\    TRJ  n  >\ 

A  recent  very  significant  paper  by  Spemann  and 
Falkenburg  (1919)  has  thrown  much  light  on  the  problem 
of  reversed  symmetry.  The  object  of  this  investigation 
was  to  discover  what  would  be  the  symmetry  relations 
in  twins  artificially  produced  by  severing  the  blastula 
or  gastrula  stage  down  the  sagittal  plane.  This  op. 
tion  effectually  isolates  the  right-  and  left-hand  primordia 


176  THE  PHYSIOLOGY  OF  TWINNING 

of  a  blastoderm.  The  amphibian  embryo  has  a  high  capa- 
city for  regeneration  and  hence  a  very  large  proportion  of 
the  half-embryos  regenerated  so  as  to  produce  each  a 
whole  individual.  As  a  rule,  however,  the  regenerated 
half  did  not  grow  so  rapidly  nor  differentiate  so  completely 
as  the  older  half.  The  result  is  that  the  right  side  of 
the  embryo  derived  from  the  left-hand  piece  and  the 
left  side  of  the  embryo  derived  from  the  right-hand 
piece  were  usually  imperfect  in  various  ways.  We  may 
speak  of  the  regenerated  side  of  both  twins  as  the  inner 
side.  Now  it  happens  that  on  the  inner  side  the  append- 
ages are  always  smaller,  the  eyes  sometimes  smaller, 
and  the  body  musculature  much  less  developed.  As  a 
consequence  of  the  lack  of  body  musculature  the  bodies 
of  the  twin  embryos  are  often  concave  on  the  inner  side 
and  may  be  even  spirally  coiled.  In  this  way  it  often 
happens  that  structures  on  the  two  outer  sides  are  similar 
and  those  on  the  two  inner  sides  equally  similar.  This 
is  a  sort  of  mirror-image  asymmetry  that  might  be 
readily  explained  as  the  result  of  imperfect  regeneration 
of  the  side  that  had  been  lost;  but  it  is  not  this  rather 
obvious  sort  of  mirror-imaging  that  attracted  the  interest 
of  these  authors.  The  really  striking  discovery  was  that 
out  of  the  thirty  right-hand  pieces  that  survived  exactly 
half  grew  into  larvae  that  showed  situs  inversus  viscerum; 
the  other  half  showed  normal  left-hand  asymmetry. 
This  discovery  is  rendered  even  more  striking  by  the 
fact  that  out  of  twenty-five  surviving  left-hand  pieces 
none  showed  definite  reversed  or  right-hand  symmetry. 
One  left-hand  piece  there  was  in  which  a  slight  irregu- 
larity of  the  heart  was  noted,  that  might  have  been 
interpreted  as  a  case  of  minimal  reversed  symmetry. 


SYMMETRY  REVERSAL  AND  MIRROR-IMAGING    177 

It  would  be  very  remarkable  if  the  left-hand  pieo 
developed  into  larvae  with  right-hand  situs,  for  the 
species  shows  typically  only  left-hand  asymmetrj 
in  man  and  the  fishes.  There  is,  however,  some  1 
to  emphasize  the  frequency  of  situs  inversus  in  the 
right-hand  pieces.  Even  those  right-hand  twins  that 
were  not  classed  as  definite  cases  of  situs  inversus 
showed  what  we  may  legitimately,  I  think,  consider  as 
cases  of  partial  situs  inversus,  since  they  were  scarcely 
at  all  asymmetrical  (i.e.,  lacked  the  normal  left-hand 
asymmetry)  or  had  only  a  slight  degree  of  left-hand 
asymmetry.  On  the  whole,  then,  we  may  say  that,  as  a 
rule,  the  right-hand  pieces  show  more  or  less  situs  inversus 
or  the  asymmetry  which  we  would  expect  to  find  in  an 
individual  derived  from  the  right-hand  primordium  of  a 
blastoderm  that  had  already  established  its  axis  of 
symmetry. 

Spemann,  in  an  elaborate  discussion  of  the  causi 
of  situs  inversus  in  these  experimental  twins,  seems 
inclined  to  refer  the  normal  specific  (left-hand)  asym- 
metry back  to  certain  asymmetric  relations  of  a  mole 
ular  sort  in  the  egg.  He  presents  as  an  analogy  to 
the  foregoing  experiments  certain  facts  brought  out  by 
Przibram  in  connection  with  crystals.  Certain  asym- 
metrical crystals,  after  one  side  has  been  injured  by 
cutting,  rebuild  themselves  so  as  to  be  the  mirror-image 
duplicates  of  the  typical  crystal.  It  Is  suggested  by 
Przibram  that  this  reversal  of  asymmetry  Is  due  to 
reversal  of  the  microstructure  of  the  crystals,  possibly 
involving  actual  molecular  changes.  <  >ne  finds  such  an 
analogy  extremely  attractive  and  almost  unescapable 
whenever  an  attempt  is  made  at  an  ultimate  analysis 


178  THE  PHYSIOLOGY  OF  TWINNING 

of  organic  asymmetry.     As  yet,  however,  the  theory  far 
outstrips  the  facts. 

Using  this  idea  as  a  working  hypothesis  we  might 
readily  conceive  of  a  condition  in  which,  because  of  the 
fundamental  molecular  asymmetry  characteristic  of  a 
group  of  animals,  one  side  of  the  body  (the  left  in  verte- 
brates) is  the  superior  side  in  a  physiological  sense. 
This  side  normally  grows  more  rapidly  so  as  to  produce 
curvatures  or  unilateral  hypertrophies  of  certain  median 
organs;  or  else  certain  other  organs,  ordinarily  paired, 
grow  only  on  the  left  side.  The  right  side  is  the  inferior 
side  and,  in  the  presence  of  the  left  or  superior  side,  is  kept 
in  some  sort  of  subordination.  If  some  agency  lowers 
the  dominance  of  the  superior  side  the  inferior  side  might 
become  independent  and  might  develop  its  own  sym- 
metry relations  without  hindrance  from  the  superior  side. 

Let  us  examine  the  facts  of  situs  inversus  in  Triton 
twins  in  the  light  of  this  theory.  These  isolated  right 
halves  will,  during  the  period  of  regeneration,  have  a 
superior  side  of  their  own,  the  outer  or  right  side,  and 
this  side  will  more  or  less  completely  dominate  the 
regenerating  left  side.  Thus  the  molecular  asymmetry 
is  doubtless  established  in  a  reversed  direction.  In 
some  right-hand  pieces,  however,  there  may  be  sufficient 
of  the  left-hand  material  present  to  set  the  molecular 
symmetry  more  or  less  completely  to  the  left,  thus  pro- 
ducing the  typical  situs  of  the  species.  This  theory  of  a 
superior  and  an  inferior  side  in  vertebrates  is  in  accord 
with  many  different  facts.  We  know,  for  example,  that 
man  is  typically  right  handed  because  the  motor  centers 
of  the  right-hand  musculature  lie  chiefly  on  the  left  side 
of  the  brain.     The  abnormal  condition  of  hemihypertro- 


SYMMETRY  REVERSAL  AND  MIRROR  IMAGING 

phy  in  man  strikes  the  right  or  inferior  side  in  about 
three-quarters  of  the  cases.  Amphioxus  is  quite  asym 
metrical,  with  many  structures  on  the  Left  or  superior 
side  that  are  lacking  on  the  right.  We  shall  now 
amine  a  parallel  situation  among  the  echinoderms  where 
unilateral  asymmetry  is  very  marked  indeed,  but  still 
capable  of  experimental  reversal. 

REVERSED    SYMMETRY   IN   ECHINODERM    LARVAE 

In  sharp  contrast  with  the  vertebrates,  in  which 
unilateral  asymmetry  is  at  best  only  slight,  stand  the 
echinoderms  in  which  certain  structures  of  the  left  side 
grow  so  much  more  rapidly  than  those  of  the  right  that 
they  almost  crowd  out  the  corresponding  right-hand 
structures  altogether.  The  early  echinoderm  larva  is, 
at  least  morphologically,  bilaterally  symmetrical,  and 
it  is  only  in  relatively  advanced  larval  stages  that  the 
left  side  begins  to  show  its  superiority  over  the  right. 
During  the  development  of  the  coelomic  pouches  a 
left-hand  hydrocoele  appears,  which  has  no  counterpart 
on  the  right  side.  This  hydrocoele  is  the  primordium 
of  the  madreporic  pores  and  pore-canals  and  of  the  radial 
water-vascular  system.  In  sea  urchins  tin-  presence  of 
the  hydrocoele  also  stimulates  the  development  <>t  the 
so-called  amniotic  invagination  on  the  left  side,  which 
has  no  counterpart  on  the  right. 

SPORADIC    INSTANCES    OF    BILATERALITV    IN     ECHINODE] 
LARVAE   AND   THEIR   SIGNTFICANCl 

One  of  the  favorite  anomalies  «>t'  the  invertebrate  em 
bryologist  is  the  occasional  larva  in  which  the  hydrocoele 
and  its  accompaniments  appear  on  the  right  side  as  well 


180  THE  PHYSIOLOGY  OF  TWINNING 

as  on  the  left.  This  appearance  of  bilateral  symmetry 
in  the  occasional  larva  has  been  interpreted  by  McBride 
and  others  as  a  reversion  to  an  ancestral  state,  and  is 
believed  strongly  to  support  the  theory  that  the  echino- 
derms,  at  present  a  radially  symmetrical  group,  have 
been  derived  from  a  bilaterally  symmetrical  ancestor 
through  the  gradual  suppression  of  the  coelomic  struc- 
tures of  the  right  side. 

Recently  the  writer  (1921,  a),  while  engaged  upon 
the  experimental  production  of  twins  in  the  starfish 
Patiria  miniata  discovered  a  culture  of  advanced  bipen- 
nariae  in  which  the  majority  of  individuals  had  both 
right  and  left  madreporic  pores  and  pore-canals.  A 
typical  specimen  of  this  culture  is  shown  in  Figure  64. 
These  specimens  were  nearly  always  strictly  bilaterally 
symmetrical,  but  a  few  showed  a  slightly  less  perfect 
condition  of  the  madreporic  canal  or  pore  on  the  right 
side  than  on  the  left.  It  should  be  emphasized  that  these 
bilaterally  symmetrical  larvae  were  found  in  a  crowded 
culture  in  which  development  had  taken  place  under 
somewhat  subnormal  conditions.  It  previously  had 
been  found  that  when  large  numbers  of  eggs  were  allowed 
to  develop  in  a  single  vessel  there  always  occurred  a 
considerable  number  of  various  types  of  twins.  When, 
therefore,  along  with  a  considerable  number  of  twin 
larvae,  there  appeared  a  great  many  of  these  bilaterally 
symmetrical  larvae,  it  was  natural  to  look  upon  this 
condition  as  a  kind  of  twinning.  The  idea  then  was  that 
any  doubling  of  normally  single  structures  might  be 
interpreted  as  twinning,  and,  therefore,  animals  with 
paired  madreporic  pores  and  pore-canals,  instead  of  the 
single  pores  and  canals,  were  essentially  twins.     I  now 


SYMMETRY  REVERSAL  AND  MIRROR-IMAGING    [8] 

look  upon  this  situation  in  a  somewhat  different  Light. 
While  still  holding  that  the  appearance  of  bilaterally 
symmetrical  larvae  is  a  sort  of  twinning  phenomenon, 
I  believe  that  it  is  really  a  phase  of  symmetry  n  il. 

In  the  typical  larva,  with  but  one  hydroc<nl<    comp 


Fig.  64. — Ventral  view  of  an  advanced  bipennaria  larva, 
observed  by  me,  of  the  starfish  Patiria   miniata.     The  water 
pore-canals,   and  hydrocoele  vesicles  arc  symmetrically  developed 

both  sides.     (Original.) 


182  THE  PHYSIOLOGY  OF  TWINNING 

on  the  left-hand  side,  it  seems  quite  evident  that  there 
is  a  pronounced  superiority  of  the  left  side,  for  the  left 
side  develops  many  important  structures  that  normally 
do  not  appear  on  the  right  side  at  all,  but  are  certainly 
potentially  represented  on  that  side. 

We  are  driven  to  the  conclusion  that  the  appearance 
and  rapid  development  of  the  hydrocoele  structures  on 
the  left  side  inhibits  in  some  way  the  development  of 
equivalent  structures  on  the  right.     It  is  well  known  that 
an  actively  growing  region  may  inhibit  the  development 
of  other  actively  growing  regions,  as  when  a  terminal 
bud  in  a  plant  inhibits  for  some  distance  back  of  it  the 
growth  of  lateral  buds.     If,  however,  such  a  terminal 
bud  is  cut  off,  or  if  in  some  way  its  rate  of  growth  is 
retarded,  the  lateral  buds  are  free  to  go  ahead.     In  some 
such  way  as  this   I  would  interpret  the  physiology  of 
the  bilaterally   symmetrical   condition   in   the   starfish 
larvae  above  described.    Normally  the  left  side,  especially 
in  the  region  where  the  hydrocoele  develops,  has  a  more 
rapid  rate  of  development  than  has  a  similar  region  on 
the  right  side.     The  left  side  may  be  compared,  therefore, 
to  a  terminal  bud  which  inhibits  the  development  of 
equivalent  growths  on  the  right  side.     If,  however,  the 
developmental  rate  of  the  embryo  is  checked  at  some 
critical  period  when  the  asymmetry  of  the  two  sides  is 
being  established,  the  discrepancy  between  the  two  sides 
fails   to   appear   and    they  start   their  hydrocoele  dif- 
ferentiation simultaneously  as  twin  hydrocoeles,  neither 
of  which  is  dominant  over  the  other  and  therefore  neither 
one  is  inhibited. 

McBride  (1918)  succeeded  in  producing  a  considerable 
number  of  bilaterally   symmetrical  larvae  of  Echinus 


SYMMETRY  REVERSAL  AND  MIRR(  )R  I  M  \<  .1  \<  i 

miliaris  by  subjecting  the  eggs  and  early  larvae  to  hyper 
tonic  sea  water.  In  spite  of  the  fact  that  the  bilateral 
condition  is  experimentally  produced  under  abnormal 
conditions,  he  adheres  to  his  view  that  the  development 
of  a  right-hand  hydrocoele  in  addition  to  the  norma] 
left-hand  one,  is  lCan  indication  that  the  common  bilateral 
ancestor  of  the  Echinodermata  had,  corresponding  to 
the  hydrocoele,  a  paired  organ  equally  developed  on 
both  sides  of  the  body,  and  that,  whilst  the  organ  on  the 
left  side  became  further  developed  until  it  grew  to  be 
the  water-vascular  system  and  its  appendages,  the  organ 
on  the  right  side  dwindled  and  disappeared.' ' 

It  seems  more  logical  to  me  to  conceive  of  the  ance 
tral  echinoderm  as  a  simple  bilateral  organism  which 
through  some  mutational  change  in  the  germinal  proto- 
plasm acquired  an  asymmetry  which  enabled  only 
the  left-hand  hydrocoele  to  develop.  This  originated 
the  first  step  in  radial  symmetry  which  culminated 
in  the  condition  now  present.  When  both  sides  develop 
hydrocoeles  we  have  a  condition  equivalent  to  twinning, 
which  can  hardly  be  thought  of  as  an  ancestral  remi- 
niscence. Moreover,  we  now  know  that  instances  ol 
reversed  symmetry  are  almost  as  common  as  cases  oi 
bilateral  symmetry  in  echinoderm  larvae,  and  it  would 
be  impossible  to  consider  a  right-handed  individual 
ancestral  to  a  left-handed  one. 

CASES    OF    SYMMETRY   REVERSAL    1\ 
ECHINODERMS 

A  significant  paper  by  Oshima  (1921)  has  recently 
appeared  in  which  that  writer  gives  an  account  ol  the 

discovery  in  certain  laboratory  culture-  of  /.■  hinus  null- 


184 


THE  PHYSIOLOGY  OF  TWINNING 


aris  of  "a  number  of  abnormal  plutei  which  had  the 
hydrocoele  developed  on  the  right  side  instead  of  in  its 
normal  position  on  the  left  side."  He  calls  attention  to 
the  fact  that  several  other  authors  had  previously  noted 
instances  ol  reversed  symmetry  in  echinoderms.     Runn- 

strom  (191 2)  had  found  this 
condition  in  two  individuals 
of  Strongylocentrotus  lividus; 
Miiller  (1850)  had  long  ago 
described  auriculariae  with 
hydrocoele  on  the  right  side 
only ;  while  Mortensen  had 
observed  two  plutii  of 
Ophionotus  hexactis  show- 
ing similar  conditions. 

The  purpose  of  Oshima's 
experiments  was  to  repeat 
McB ride's  method  of  pro- 
ducing larvae  with  double 
hydrocoeles.  Using  the 
latter's  procedure  he  ob- 
tained about  10  per  cent 
of  larvae  exhibiting  situs 
inversus  (Fig.  65)  and  about 
2  per  cent  with  double 
hydrocoeles.      Curiously 


Fig.  65. — Ventral  view  of  a 
pluteus  larva  of  the  sea  urchin, 
Echinus  miliaris,  showing  com- 
plete reversal  of  asymmetry  in  the 
hydrocoele,  which  appears  on 
the  right  side  only  instead  of  on 
the  left,  the  situs  typical  for  the 
species.     (From  Oshima.) 


enough,  however,  the  control  cultures,  which  had  not 
been  treated  with  hypertonic  sea  water  showed  the 
same  percentage  of  anomalous  forms.  It  must  not  be 
forgotten,  however,  that  the  cultures  were  reared  in 
artificially  mixed  sea  water  and  that  the  food  was  scanty. 
These   conditions   were   sufficient   to   account   for   the 


SYMMETRY  REVERSAL  AND  M  [RROR-IMAGING 

peculiar  condition.  Oshima's  own  explanation  of  the 
condition  is  as  follows:  ''The  growth  of  the  normally 
developing  hydrocoele  might  have  been  arrested  from 
some  cause  and  the  right  anterior  coelom,  to  i  ompensate 
this  defective  development,  produced  a  new  hydrocoele 
on  the  right  side."  To  me  the  condition  is  simply  an 
exaggeration  of  that  in  which  bilaterally  symmetrical 
hydrocoeles  are  produced.  In  some  way.  as  the  result 
of  abnormal  growth  conditions,  the  development  oi  the 
left-hand  hydrocoele  is  inhibited,  while  the  right-hand 
hydrocoele  becomes  physiologically  isolate*]  and  begins 
to  grow  before  the  left  has  sufficiently  recovered  to 
resume  development.  The  right-hand  hydrocoele  now 
inhibits  the  left  because  it  has  a  higher  rate  of  develop- 
ment, and  is  therefore  the  superior  side.  Thus  we  haw- 
before  us  a  case  of  complete  symmetry  reversal  experi- 
mentally induced. 

A   GENERAL   THEORY   OF   THE   PHYSIOLOGY 
OF    SYMMETRY   REVERSAL 

Any  theory  of  symmetry  reversal  must  be  founded 
upon  a  correct  understanding  of  the  physiological  status 
that  exists  in  a  perfectly  bilaterally  symmetrical  organ- 
ism. I  look  upon  such  an  organism  as  the  resultant  oi 
an  intimate  interplay  and  co-operation  of  two  systems  oi 
primordia  which  are  in  focus  upon  a  single  median  plane. 
a  plane  which  is  equivalent  to  a  sagittal  plane  of  the  adult 
organism.  The  two  antimeric  halves  are  in  ;i  sense  twin 
individuals  with  a  common  apical  plane  So  close  is 
the  co-operation  or  integration  of  the  two  halv< 
however,  that  one  half  influences  tin-  opposite  ball  in 
such  a  way  that  equivalent  structures  appear  in  the  i 


i86 


THE  PHYSIOLOGY  OF  TWINNING 


halves  as  mirror-images  of  each  other,  just  as  the  two 
halves  of  a  symmetrical  crystal  are  mirror-images. 
If,  in  our  hypothetical  perfectly  bilaterally  symmetrical 
organism,  the  two  halves  were  to  be  physically  or 
physiologically  isolated,  we  would  expect  exactly  equiva- 
lent twins,  for  each  half-primordium  would  regenerate 
in  such  a  way  as  to  reproduce  the  exact  bilateral  sym- 
metry present  in  the  original  individual  of  which  they 
are  parts. 

As  a  matter  of  fact,  however,  a  certain  amount  of 
unilateral  asymmetry  appears  to  be  characteristic  of 
most  organisms.  In  some  organisms  such  asymmetry 
is  very  pronounced,  as  in  echinoderms  and  in  gastropod 
molluscs,  while  in  other  organisms  it  is  much  less  pro- 
nounced, as  in  vertebrates  and  in  many  arthropods. 
Whether  the  unilateral  asymmetry  affects  many  organs 
or  a  few,  whether  the  extent  of  the  asymmetry  be  great 
or  little,  the  basis  of  the  asymmetry  seems  to  be  one 
involving  a  physiological  superiority  of  one  side  or  the 
other.  In  last  analysis  the  difference  between  the  two 
sides  may  be  reduced  to  terms  of  rate  of  fundamental 
vital  activity,  probably  measurable  in  terms  of  rate  of 
oxidation.  The  result  is  that  one  side  develops  rather 
more  rapidly  than  the  other,  especially  in  connection 
with  certain  structures  that  arise  near  the  median  axis 
or  mirror  plane.  Thus,  in  vertebrates,  the  left  side  of 
the  stomach,  of  the  heart,  and  of  other  median  structures 
grows  more  rapidly  than  the  right;  and  certain  other 
structures,  such  as  swim-bladder  in  fishes  and  left 
aorta  in  mammals,  appear  only  on  the  left  and  not  on 
the  right.  Similarly,  in  echinoderms  the  left  side  of  the 
larva  develops  more  rapidly  than  the  right,  and  certain 


S YMMETRY  REVERSAL  AND  MIRROR-IMAGING    I 

structures,  the  hydrocoele  and  its  derivatives,  star!  to 
grow  first  on  the  left.  In  both  of  these  groups 
believe  that  the  earlier  onset  of  rapid  growth  in  certain 
left-hand  primordia  inhibits  more  or  less  completely  the 
growth  of  equivalent  structures  on  the  right.  The 
phenomenon  of  growth  inhibition  in  these  and  other 
allied  cases  is  probably  bioelectric  in  character,  [f  s<  >.  t  he 
region  of  rapid  growth  at  any  level  of  the  prim  .try  axis 
is  positive  to  regions  of  less  active  growth  and  then'  is  a 
one-way  bioelectric  current,  which  furnishes  the  medium 
of  control  of  one  part  over  another.  R.  S.  Lillie  has 
demonstrated  similar  relations  in  connection  with  metals 
in  solutions  of  electrolytes.  One  positive  or  anodic 
region  seems  to  have  an  inhibiting  effect  over  a  given 
distance  so  that  other  similarly  charged  region-  cannot 
arise  near  the  original  anodic  region.  Whatever  be  the 
ultimate  physiology  of  the  inhibition  exercised  over  one 
growing  region  by  another  more  actively  growing  region, 
the  actual  fact  of  inhibition  is  beyond  question. 

If  now  we  in  some  way  break  down  the  co-operation 
of  the  two  half-primordia  destined  to  form  the  bilateral 
halves  of  a  single  individual,  the  two  halves  become  more 
or  less  completely  independent,  and  twinning  result-. 
If  the  twins  are  separated  down  the  whole  axis,  or  ii 
they  are  separated  as  far  back  as  the  posterior  vnd  o\ 
the  body  cavity,  the  two  severed  halves  will  each  under 
complete  regulation,  each  forming  a  whole  organism 
with  the  symmetries  and  asymmetries  characteristic  oi 
the  species.  Thus  completely  divided  human  twins, 
armadillo  twins,  and  fish  twins  rarely  it  ever  show  situs 
inversus  viscerum,  but  always  possess  the  left  hand 
asymmetry  characteristic  of  the  speci< 


188  THE  PHYSIOLOGY  OF  TWINNING 

If,  however,  as  in  the  fishes,  the  process  of  bilateral 
fission  happens  to  halt  at  a  certain  definite  level,  where 
the  alimentary  tract  is  divided  pretty  well  back  of  the 
stomach,  but  remains  single  in  a  considerable  part  of 
the  intestine,  it  seems  to  be  a  matter  of  touch  and  go 
whether  the  right-hand  component  of  the  conjoined 
twins  will  regulate  in  such  a  way  as  to  take  on  the 
normal  left-hand  asymmetry  of  the  species  (situs  solitus) 
or  will  continue  to  behave,  with  regard  to  some  of  its 
structures,  as  though  it  were  half  of  a  single  bilaterally 
symmetrical  organism .  The  condition  seems  to  me  to  be 
much  like  that  exhibited  by  bilaterally  symmetrical 
echinoderm  larvae  in  which,  the  dominance  of  the  left- 
hand  hydrocoele  having  been  reduced,  the  right-hand 
half  assumes  equivalence  and  both  develop  equally  as 
bilaterally  symmetrical  structures.  In  the  cases  in  which 
situs  inversus  viscerum  is  found  in  conjoined  twins  of  the 
fishes,  we  may  interpret  the  effect  as  due  to  a  lowering 
of  dominance  of  the  left-hand  side  over  the  right  only 
to  the  point  where  they  are  equally  dominant,  each 
being  to  a  slight  extent  influential  over  the  other.  In 
other  words  a  certain  degree  of  the  old  bilateral  integra- 
tion of  the  two  half-primordia  remains  to  express  itself 
in  the  mirror-image  relations  of  the  viscera.  When  the 
co-ordination  is  completely  broken  down  the  right-hand 
individual,  as  well  as  the  left-hand  one,  regulates  the 
normal  asymmetry  of  the  species.  There  appears  then 
to  be  a  very  delicate  equilibrium  at  some  period,  in 
connection  with  bilateral  primordia  destined  to  pro- 
duce twins,  between  a  condition  of  complete  isolation 
and  a  condition  of  partial  integration  between  the  two 
halves. 


SYMMETRY  REVERSAL  AND  MIRROR-IMAGING 

In  separate  one-egg  human  (wins  mirror  imaging 
seems  to  persist  mostly  in  certain  integumentary  stru 
turessuch  as  friction-ridge  patterns  on  index  fingers,  (X  i 
sional  reversals  in  direction  of  whirl  in  the  crown  of  tin- 
head  hair.  Yet  there  are  not  a  few  instance-  in  which 
one  twin  is  right  handed,  the  other  left  handed.  As  in 
the  case  of  fishes,  however,  the  normal  condition  in  com- 
pletely separate  twins  is  a  complete  regulation  in  both 
individuals  of  the  specific  asymmetry  of  all  structures. 
In  the  armadillos,  especially  in  the  case  of  twins  derived 
from  a  secondary  blastoderm  of  one  side,  the  incidence 
of  mirror-imaging  is  more  frequent,  there  being  nearly 
as  many  instances  in  which  some  asymmetrical  integu- 
mentary peculiarity  is  found  on  opposite  sides  of  a 
pair  of  twins  as  on  the  same  side.  Here  again  the 
equilibrium  at  the  time  of  separation  of  the  twin  pri- 
mordium  must  be  extremely  delicate  and  some  very 
minor  factor  may  decide  whether  the  two  individuals 
shall  both  show  unilateral  asymmetry  of  the  same  side 
or  whether  one  shall  be  the  mirror-image  of  the  other. 
In  these  cases  we  cannot  speak  of  symmetry  reversal 
because  we  do  not  know  with  regard  to  any  sporadic 
asymmetry  of  the  scute  pattern  what  is  the  specific 
condition  or  the  situs  soliius.  All  we  can  say  Is  that 
both  individuals  are  mirror-images  of  each  other.  In 
the  next  chapters  are  discussed  further  cases  oi  mirr 
imaging  that  must  be  taken  into  account  in  reaching 
any  final  judgment  as  to  the  causes  and  significance  ol 
symmetry  reversals. 


CHAPTER  XIII 
DOUBLE  TAILS  IN  VERTEBRATES 

Among  bilateral  animals  of  all  sorts  it  is  very  com- 
mon to  find,  instead  of  the  single  tails  or  limbs  character- 
istic of  the  species,  double  tails  or  limbs.  Such  double 
structures  are  unsually  known  as  duplicities,  but  are 
really  cases  of  local  twinning,  as  I  shall  attempt  to  show. 

Double  tails  have  been  very  frequently  described 
in  connection  with  experiments  on  regeneration  of  lost 
tails.  Not  uncommonly  one  finds  in  lizards  that  the 
regenerated  end  of  the  tail  has  grown  out  more  or  less 
completely  doubled.  Similar  results  have  been  reported 
in  connection  with  regenerated  tails  of  various  Amphibia. 

Some  years  ago  when  the  writer  (191 56)  was  en- 
gaged in  an  extensive  series  of  hybridization  experi- 
ments upon  the  bony  fishes  it  was  noted  that  in  some 
crosses,  a  considerable  percentage  of  the  hybrid  larvae 
had  double  tails.  This  was  especially  true  in  the  cross 
Tauiogolabrus  adspersus  ?  X  Stenotomus  chrysops  &  (Cun- 
ner  $  X  butterfish  s).  This  cross  furnishes  a  very 
extensive  assortment  of  monstrosities  among  which 
there  were  both  double-headed  and  double-tailed  indi- 
viduals. The  double-tailed  ones  were,  however,  very 
numerous.  Occasional  double-tailed  individuals  were 
found  in  the  various  other  crosses,  especially  in  the 
cross  between  the  eggs  of  the  butterfish  and  the  sperm 
oiFundulus  heteroclitus,  where  one  perfectly  symmetrical 
double  tail  was  found.  In  all  of  these  experiments  the 
double-tailed  condition  was  found  associated  with  various 

190 


DOUBLE  TAILS  IN  \  KR'I  l.l;K.\  I  ES  [9] 

other  abnormalities  such  as  cyclopia,  humpba<  I.,  abnor 
mal  heart.  It  seems  only  reasonable,  then,  to  infer 
that  the  same  types  of  causes  arc  responsible  for  the 
excessive  growth  seen  in  double  tail  as  arc  responsible 
for  the  defects  above  named.  In  all  cases  it  is  very 
plain  that  the  rate  of  development  from  an  early  period 
has  been  decidedly  retarded  as  compared  with  that 
seen  in  the  corresponding  pure-bred  embryos. 

DOUBLE-TAILED    GOLDFISH 

One  species  of  fish  is  characteristically  double  tailed 
even  in  nature — the  goldfish  (Cyprinus  auratus).  The 
production  of  the  double-tailed  conditions  is,  like  that 
of  other  morphological  oddities  in  these  fishes,  under  tin- 
control  of  the  breeders,  who  are  experts  in  these  matter-. 
Two  particular  kinds  of  double  tails  are  common: 
those  in  which  each  half  of  the  double  tail  is  essentially 
a  complete  tail  and  the  two  tails  lie  side  by  side,  only 
united  dorsally  at  the  point  of  their  union  with  the  body; 
(b)  those  in  which  the  two  tails  are  more  or  less  com- 
pletely fused  by  their  dorsal  margins  in  such  a  way  that 
the  double  fin  is  a  three-lobed  affair.  All  stages  of  com- 
plete and  incomplete  doubling  occur  in  any  lot  of  fishes 
derived  from  one  batch  of  eggs.  Sometimes  the  doubling 
involves  the  vertebral  column  and  sometimes  only  the 
fin  rays  or  vertebral  arches. 

The  condition  of  double  tail  does  not  seem  to  be 
definitely  heritable  in  goldfishes.  Normal  parents  pro 
duce  many  offspring  with  double  tails  and  double  tailed 
individuals  produce  many  normals.  There  is  inherited, 
it  seems,  merely  a  high  degree  of  susceptibility  to  the 
conditions  responsible  for  doubling.  What  these  are  we 
shall  now  inquire. 


192  THE  PHYSIOLOGY  OF  TWINNING 

THE   CAUSES    OF   DOUBLE   TAILS 

Tail  doubling  is  in  my  opinion  a  phenomenon  very 
similar  to  head  doubling,  but  is  probably  characteristic 
of  a  later  developmental  period. 

Thanks  to  the  recent  experiments  of  Dr.  Hyman 
(192 1)  we  now  know  that  the  fish  embryo,  during  a 
relatively  early  germ-ring  stage,  forms  the  rudiments  of 
a  tail-bud,  the  equivalent  of  the  Knopf  of  Kopsch.  This 
posterior  region  of  the  primary  axis  has  from  an  early 
time  a  very  high  relative  susceptibility  to  inhibiting 
agents,  such  as  anaesthetics,  lack  of  oxygen,  cold.  Even 
in  presomite  stages  of  the  embryo  there  is  present  a 
double  gradient,  similar  to  that  in  annelid  worms,  with 
points  of  high  activity  and  susceptibility  at  the  anterior 
and  at  the  posterior  ends  and  with  gradients  of  suscepti- 
bility running  both  backward  and  forward.  After  the 
anterior  parts  of  the  axis  are  fully  established  and  have 
undergone  considerable  differentiation,  the  posterior  end 
of  the  axis  remains  an  actively  growing  region.  It  is 
evident  that  after  a  period  of  retardation  this  actively 
growing  tail-bud  region  undergoes  bilateral  fission  in 
order  to  form  double  tails.  Abundant  evidence  is  at 
hand  indicating  that  the  twinning  process  of  the  tail  is 
due  primarily  to  a  slowing-down  of  the  developmental 
rate  so  that  the  two  bilateral  halves  lose  their  co- 
ordination and  proceed  independently,  each  regulating 
for  itself  a  bilateral  symmetry  more  or  less  complete. 
Mirror-imaging  is  under  these  conditions  quite  the 
expected  thing,  and  the  expectation  is  always  realized. 
Thus  we  see  that  twinning  of  the  tail  is  extremely  like 
twinning  of  the  head  and  body,  and  doubtless  depends 
on  the  same  factors  operating  at  a  different  time. 


CHAPTER  XIV 


TWINNING  (DUPLICITY)  IN  LIMBS 

As  long  ago  as   1894  Bateson,  in  his  classic  work 
Materials  for  the  Study  of  Variation,  devoted  two  chapters 
to    "  Supernumerary   Appendages   in    Secondary    Sym- 
metry."    A  representative  series  of  instances  of  super- 
numerary appendages  such  as  antennae,  palpi,  and  L< 
is  described,  involving  many  groups  of  insects,  Crustacea, 
and  vertebrates.     These  extra  appendages  may  be  cither 
entirely  separate  outgrowths  near  the  normal  appendu: 
or,  as  is  the  case  in  the  majority  of  instances,  they  may 
occur  as  outgrowths  from 
an    appendage — -such    as 
extra  legs  growing  from 
normal  legs.     In  both 
types  of   cases  the  sym- 
metry of  the  supernum- 
erary appendage  appears 
to  bear  a  definite  relation 
to  that  of  the  neighbor- 
ing appendage  or  to  the 
one  upon  which  it  grows. 
It  is  quite  common  to  find 
that    the   supernumerary 
appendage  growing  from 
another  appendage  is 
itself  a    twin   appendage 
in  which  the  two  parts  are  mirror-images  of  each  other 
A  very  pretty  case  of  this  relation  is  seeo  in  Figure 

193 


Fig.  66. — An  example  of  Limb 
duplicity  in  an  inse<  t.  Pi 
miihlfddii.  The  component  to  the 
right  is  the  normal  tarsus.  Hie 
extra  tarsus  on  the  left  IS  dup 
and  shows  mirror-image  symmetry. 
(After  Bateson.) 


194  THE  PHYSIOLOGY  OF  TWINNING 

an  anomalous  leg  of  a  beetle.     On  the  basis  of  a  large 

number  of  such  instances  Bateson  is  able  to  set  down 

certain  rules  of  symmetry: 

When  extra  appendages,  arising  from  a  normal  appendage, 
are  thoroughly  relaxed  and  extended,  the  following  rules  will  be 
found  to  hold  good  with  certain  exceptions  to  be  hereafter  specified : 

I.  The  long  axis  of  the  normal  appendage  and  the  two  extra 
appendages  are  in  one  plane:  of  the  two  extra  appendages  one  is 
therefore  nearer  to  the  axis  of  the  normal  appendage  and  the  other 
remoter  from  it. 

II.  The  nearer  of  the  two  appendages  is  in  structure  and  position 
formed  as  the  image  of  the  normal  appendage  in  a  plane  mirror 
placed  between  the  normal  appendage  and  the  nearer  one,  at  right 
angles  to  the  plane  of  the  three  axes;  and  the  remoter  appendage  is 
the  image  of  the  nearer  in  a  plane  mirror  similarly  placed  between  the 
two  extra  appendages. 

The  symmetry  between  the  nearer  double  member 
and  the  normal  appendage  may  be  called  primary 
symmetry  and  that  between  the  two  twinned  members, 
secondary  symmetry.  The  terms  major  and  minor  sym- 
metry are  sometimes  used  quite  synonomously  with  these. 

In  a  book  of  the  present  scope  it  would  neither  be 
desirable  nor  feasible  to  enter  into  an  exhaustive  survey 
of  the  elaborate  literature  on  duplicities  and  symmetries 
in  limbs.  Hence  we  shall  confine  ourselves  to  a  few 
selected  phases  of  the  subject  that  appear  to  be  especially 
helpful  in  our  attempt  to  analyze  the  phenomena  of 
organic  symmetry  and  symmetry  reversal. 

EXPERIMENTAL  PRODUCTION   OF   DOUBLE 
LIMBS   IN   AMBLYSTOMA   LARVAE 

Harrison  (1920)  has  succeeded  in  producing  a  large 
number  of  double  and  triple  limbs  in  the  larvae  of 
the  common  salamander  Amblystoma  by  transplanting 


TWINNING  (DUPLICITY)  IN  LI. Ml 

limb-buds  at  an  early  stage  to  another  part  of  the  body. 
He  used  a  small  circular  punch  by  means  of  which  he 
was  able  to  cut  out  a  little  circle  of  the  body  wall  of  tin- 
embryo  from  the  region  known  to  contain  the  primordia 
of  the  fore  limbs.  These  little  circles  of  embryonic 
tissue  wrere  then  placed  in  various  positions  (hind  part 
before,  upside  down,  and  diagonally)  in  wound  beds  of 
the  correct  size  that  had  been  cut  out  of  the  body  wall 
at  other  places  than  those  normal  for  the  development 
of  limbs.  Sometimes  a  right-hand  limb-bud  was  put 
on  the  left  side,  and  vice  versa,  in  the  various  positions 
stated  above.  These  small  transplanted  limb  primordia 
produce  limbs  in  their  new  positions,  but  they  show 
varied  results  according  to  the  side  on  which  they  arc- 
transplanted  and  the  position  in  which  the  pieces  were 
placed.  In  general  it  may  be  said  that  a  graft  tends  t<> 
produce  a  limb  with  the  same  symmetry  relations  that 
it  would  have  had  if  left  where  it  originally  was,  but 
that  there  is  more  or  less  complete  symmetry  reversal 
in  some  cases.  The  normal  limb  grows  backward,  but 
if  a  limb-bud  is  transplanted  hind-part-before  the  limb 
will  grow  forward.  The  palmar  surface  of  the  limb 
tends  to  form  on  the  side  turned  toward  the  body  of 
the  animal  and  the  ulnar  border  tends  to  be  dorsal. 
Harrison  says: 

The  above  circumstances  determine  the  asymmetry  <»t  the 
limb  as  follows:  when  the  dorso-ventral  axis  is  nut  Inverted,  the 
original  prospective  asymmetry  persists;  when  the  axis  is  inverted, 
the  asymmetry  is  reversed.  In  more  general  term^:  the  asyi 
metry  of  the  limb  is  determined  by  two  factors,  the  polarization 
of  the  anterio-posterior  axis  of  the  limb-bud  and  the  orientation 
of  the  limb-hud  with  respect  to  the  dorso  ventral  polarization 
its  organic  environment. 


196  THE  PHYSIOLOGY  OF  TWINNING 

This  reversal  of  symmetry  reminds  one  of  the  sym- 
metry reversal  seen  in  twins  and  double  monsters.  It 
will  be  remembered  that  in  those  cases  there  was  a 
delicate  equilibrium  between  the  internal  factors  and 
the  external  factors.  Sometimes  the  internal  factors, 
the  inherent  tendency  for  a  twinned  right-hand  com- 
ponent to  assume  the  characteristic  left-handed  asym- 
metry of  the  species,  prevails;  sometimes  the  external 
factor,  the  close  proximity  of  the  left-hand  component 
and  a  tendency  to  integrate  with  it,  causes  the  right- 
hand  component  to  act  as  though  it  were  merely  a 
right-hand  mirror-image  duplicate  of  the  left-hand 
component,  and  we  have  symmetry  reversal. 

Our  interest  in  Harrison's  work  is,  for  the  moment, 
limited  to  the  phenomenon  of  limb-doubling.  Double 
and  triple  limbs  arise  frequently  from  the  transplanted 
limb-buds.  These  are  of  all  grades  of  completeness  and 
occur  under  a  great  variety  of  different  experimental 
conditions.  They  occur  most  commonly,  however,  when 
the  buds  are  transplanted  farthest  away  from  their  nor- 
mal positions.  In  most  cases  of  double  limbs  Harrison 
is  able  to  distinguish  an  original  (primary)  and  a  second- 
ary limb  which  he  conceives  of  as  arising  as  a  lateral 
outgrowth  from  the  primary. 

This  symmetry  relation  is  evidently  an  extremely 
fundamental  phenomenon  and  in  many  respects  reminds 
one  of  bilateral  symmetry  and  of  symmetry  reversal  in 
conjoined  twins.  While  in  conjoined  twins  there  are  many 
exceptions  to  mirror-imaging,  due  to  a  regulation  on  the 
part  of  the  partially  separated  structures  back  to  the 
specific  asymmetry,  there  are  in  the  case  of  these  double 
limbs  very  few  exceptions  to  the  mirror-image  rule.  These 


TWINNING  (DUPLICITY)  IN  LIMBS  [97 

exceptions  are,  however,  significant,  since  they  represent 
the   equivalent   of   the   failures   to   show   mirror-ima| 
symmetry  in  twins  and  are  doubtless  due   to  similar 
causes.     Bateson  notes  two  clear-cut  cas        >f  doubled 
limbs  in  beetles  in  which  the  double  appendage  branching 
from   the  single  normal  appendage  has  no   symmetry 
relation  to  the  single   appendage,  but  the   twin  par 
are  quite  distinctly   symmetrical   between   themselv< 
This  is  really  a  breach  of  the  first  rule  of  Bateson,  that 
"the  long  axis  of  the  normal  appendage  and  the  t 
extra  appendages  are  in  the  same  plane."     Various  other 
authors  have  cited  occasional  exceptional  cases,  but  it 
must  not  be  forgotten  that  the  symmetry  rules  of  1  m 

hold  with  respect  to  a  very  high  percentage  of  all  cases. 

The  analysis  of  symmetry  relations  in  reduplicated 
limbs  of  Ambly  stoma,  as  given  us  by  Harrison,  has  been 
rendered  vastly  more  difficult  than  need  be  owing  to 
the  complexity  of  experimental  procedures.  Had  the 
author  been  interested  primarily  in  the  study  of  double 
limbs  and  their  symmetry  relations  he  could  have 
eliminated  a  great  many  complicating  factors,  such 
the  changing  of  grafts  from  right  to  Left  side  or  the 
turning  of  grafts  upside  down.  Under  the  circumstano 
it  appears  to  me  remarkable  that  so  definite  a  result 
was  obtained,  and  we  need  hardly  despair  about  the 
possibility  of  clearing  up  this  problem  If  efforts  should 
be  directed  definitely  to  that  end.  ( lertain  facts  may  be 
gleaned  from  a  survey  of  Harrison's  work  that  aid  us 
in  our  present  analysis:  (a)  The  broad  axis  oi  the  limb 
is  at  right  angles  to  the  sagittal  plain-  of  the  body, 
(b)  The  ulnar  border  of  the  limb  is  dorsal  and  the  radial 
border  ventral,  i.e.,  the  little-finger  side  of  the  hand  is 


1 98  THE  PHYSIOLOGY  OF  TWINNING 

dorsal  and  the  thumb  side  is  ventral,  (c)  The  palm 
surface  of  the  limb  is  posterior  and  the  back  of  the  hand 
is  anterior. 

With  these  points  of  orientation  in  mind,  we  are  now 
in  a  position  to  compare  and  contrast  the  twinning 
situation  as  it  presents  itself  in  conjoined  twins  and  in 
double  limbs.  In  conjoined  twins  we  found  that  the 
organic  symmetry  relations  were  influenced  only  by 
internal  factors  so  that  each  bilateral  half  was  situated 
in  a  position  exactly  equivalent  to  that  of  the  other. 
In  a  limb-bud,  however,  the  environmental  relations  of 
the  body  to  which  the  limb-bud  belongs  clearly  exercise 
an  influence  upon  the  symmetry  of  the  limb.  If  a 
limb-bud  were  to  be  transplanted  to  the  median  dorsal 
region,  so  that  its  dorsal  half  fell  on  the  right  and  its 
ventral  half  on  the  left  of  the  primary  axis  of  the  em- 
bryo, I  suspect  there  would  grow  a  perfectly  bilaterally 
symmetrical  limb  with  no  difference  between  radial 
and  ulnar  sides  and  no  difference  between  little  finger 
and  thumb.  Growing  as  it  does,  however,  the  limb-bud 
cannot  be  bilaterally  symmetrical  because  the  ulnar 
border  of  the  limb  (little-finger  side)  is  dorsal,  and 
therefore  has  a  higher  rate  of  metabolism  than  has  the 
radial  or  thumb  side.  Physiologically  the  ulnar  side  is 
the  superior  side.  There  is  therefore  an  asymmetry 
quite  similar  to  that  in  the  whole  body  of  such  animals 
as  the  echinoderms. 

If  the  superiority  of  the  ulnar  side  should  be  broken 
down  in  any  way  we  might  expect  to  get  an  equivalence 
of  superior  and  inferior  sides  something  like  that  seen 
in  conjoined  twins  with  mirror-image  symmetry,  or  like 
the  starfish  larvae  with  paired  hydrocoele  structures. 


TWINNING  (DUPLICITY)  IX  LIMBS 


199 


This  is,  I  believe,  just  what  happens  when  a  limb-bud 
undergoes  simple  twinning.  In  such  a  twinned  limb 
the  mirror  plane,  as  in  other  cases  of  symmetry  reversal, 
is  adjacent  to  the  inferior  or  radial  side  of  each  com- 


A.DU 


MP2(UJ 


MP^R) 


Fig.  67. — Diagram  showing  mode  of  limb  reduplication  (twinnii 
in  Amblystoma.     PR,   primary  limb;    P.  1)1',   posterior   reduplicating 
member;    A.DU,   anterior   reduplicating  member;     Ml',.  R  ,    primary 
(radial)  mirror  plane;    MP2{U),  secondary  (ulnar)  mirror  plain-; 
first  to  fourth  digits,  respectively.     (After  Harrison.) 

ponent  of  the  double-monster  limb  and  the  superior  side 
is  away  from  the  mirror  plane  in  each  instance.  Such  a 
double  limb  as  this  is  represented  in  Figure  67,  and  should 
be  compared  first  with  a  mirror-imaged  double  monster, 
such  as  that  of  the  trout  (Fig.  (>})  and  then  with  the 
completely  double  hand  shown  on  page   202     I  ig.  71 


200  THE  PHYSIOLOGY  OF  TWINNING 

The  situation  is  greatly  complicated,  however,  when  one 
of  the  components  of  the  twinned  limb  undergoes  a 
secondary  reduplication.  The  mirror  plane  is  then  on 
the  ulnar  or  superior  side  of  the  twinned  limbs.  It 
could  not  be  otherwise  without  doing  away  with  the 
mirror-imaging  between  the  first  pair  of  twin  com- 
ponents, for  they  could  not  all  three  have  the  mirror 
plane  on  the  radial  or  inferior  side.  This  secondary 
reduplication  undoubtedly  greatly  complicates  matters 
and  renders  the  analysis  of  symmetry  reversal  extremely 
difficult.  It  is  easy  to  state  the  rule  according  to  which 
mirror-imaging  works  out,  just  as  both  Bateson  and 
Harrison  have  done,  but  it  is  not  nearly  so  easy  to  ac- 
count on  physiological  grounds  for  what  is  so  readily 
formulated. 

It  is  clear  that  the  complicating  factor  is  the  second 
step  in  limb-doubling.  If  we  could  find  a  material  in 
which  limb- doubling  was  simpler  we  could  perhaps 
obtain  a  less  complicated  situation  that  would  admit 
of  more  ready  analysis.  It  is  fortunate  that  we  have 
just  such  a  series  of  instances  of  simple  limb-doubling 
in  human  hands  and  feet. 

DOUBLE   HANDS   AND   FEET  IN  MAN 

Bateson  (1894)  has  described  and  figured  a  number 
of  significant  instances  of  hand  and  foot  anomaly  which 
seem  to  me  to  help  us  to  bring  the  phenomenon  of 
limb-doubling  into  line  with  bilateral  twinning.  These 
will  be  listed  in  a  logical  series: 

1 .  The  minimal  case  of  mirror-imaging  or  break-down 
of  dorso-ventral  asymmetry  of  the  left  hand  is  one  in 
which  the  thumb  is  essentially  the  same  as  the  little 


TWINNING  (DUPLICITY)  IN   LIMBS  201 

finger,  though  a  little  larger.      It   has  three  joints  Like 

the  little  finger  and  takes  its  origin  from  the  palm  at 
the  same  level  as  the  latter  (Fig.  68,  p.  20 

2.  We  next  have  an  instance  in  which  the  ph 
logical  isolation  of  the  two  sides  of  the  hand  was  more 
pronounced,  so  that  partial  twinning  has  occurred.     T! 

is  a  case  of  a  woman  who  had  six  digits  on  each  hand 
and  foot.     In  each  hand  the  thumb  has  three  joints  like 
a  little  finger.     There  are,  however,  sonic  irregularitii 
that  complicate  the  case. 

3.  Another  case  is  cited  of  a  hand  with  six  digits 
arranged  in  two  groups  that  were  somewhat  opposable 
to  each  other  (Fig.  69).  Digits  II,  III,  IV.  V  stand  in 
normal  position  as  the  ulnar  set,  while  the  radial  group 
consists  of  two  normal  fingers,  each  with  three  joints, 
and  neither  one  thumblike.  Here  we  have  partial 
twinning  and  partial  mirror-imaging,  but  the  radial  side 
is  still  somewhat  inferior.  This  case  seems  to  indicate 
that  in  a  hand  of  normal  type  the  thumb  stands  over 
against  the  rest  of  the  fingers  as  a  reduced  or  inferior 
half  of  the  appendage.  The  physiological  isolation 
the  wreaker  from  the  stronger  side  allows  the  thumb 
side  to  become  more  nearly  equal  to  the  little-finger 
side. 

4.  A  still  more  nearly  double  hand  is  one  described 
from  a  specimen  in  the  Harvard  Medical  School  Museum. 
The  forearm  consists  of  two  ulna  bones  instead  of  a  radius 
and  an  ulna.  In  other  word-,  the  superior  oi  these  twin 
bones  is  repeated  on  both  sides.  The  hand  oi  this 
double  arm  consists  of  seven  digits  in  two  groups,  an 
ulnar  group  of  four  very  normal  digits  and  a  radial 
thumb  group  of  three  nearly  normal  fingers    Fig    70V 


202 


THE  PHYSIOLOGY  OF  TWINNING 


5.  A  case  of  complete  double  hand  with  perfect 
mirror-image  symmetry  is  described  by  J.  J.  Murray. 
This  hand,  shown  in  Figure  71,  appears  to  have  been 
completely  bilaterally  symmetrical  and  without  thumbs, 


Figs.  68-71. — Various  degrees  of  duplicity  (twinning)  of  the  human 
hand.  Fig.  68,  a  hand  with  a  thumb  like  a  little  finger.  Fig.  69,  a  hand 
with  thumb  represented  by  two  fingers.  Fig.  70,  a  hand  with  thumb 
represented  by  three  fingers.  Fig.  71,  hand  completely  twinned  with 
the  thumb  represented  by  four  fingers  which  are  mirror  images  of  the 
fingers  in  the  radial  component.     (After  Bateson.) 

a  whole  set  of  four  fingers  having  arisen  in  the  place 
of  a  thumb.  The  two  sets  of  four  were  physiologically 
equivalent  in  use  and  value.  I  look  upon  this  case  as 
the  logical  culmination  of  the  series.  If  our  theory  is 
correct,  that  the  plane  of  symmetry  in  the  vertebrate 


TWINNING  (DUPLICITY)  IN  LIMBS  203 

hand  falls  between  the  thumb  and  the  index  fin  ad 
that  the  thumb  is  the  reduced  equivalent,  on  the  radial 
side  of  the  limb,  of  the  four  fingers  on  the  ulnar  side  of 
the  limb,  we  could  not  have  a  double  hand  with  more 
than  eight  digits.  No  instances  of  more  than  eight 
digits  have  come  to  our  attention,  and  whenever  there 
are  eight  digits  they  occur  in  two  mirror-image  sets. 

CASES  OF  SYMMETRY   REVERSAL 

In  the  case  of  double-headed  twins,  one  component, 
always  the  one  on  the  right  (the  individual  belonging 
to  the  inferior  side)  may  become  sufficiently  independent 
to  resume  the  specific  asymmetry  (situs  solitus). 
what  equivalent  changes  may  occur  in  the  case  of  double 
hands  and  feet.  An  interesting  double  right  hand  with 
eight  lingers  is  described  and  figured  by  Giraldes,  in 
which  the  outer  digit  of  the  ulnar  or  superior  side  is 
decidedly  thumblike  while  that  on  the  radial  side  is 
distinctly  a  little  finger.  This  is  like  the  complete 
reversal  of  asymmetry  seen  in  the  sea  urchin  plutei 
described  by  Oshima  (192 1).  Another  similar  case  is 
noted  by  Athol  Johnson,  but  this  time  in  a  foot  Instead 
of  a  hand.  It  is  a  case  of  a  left  foot  with  nine  toes, 
one  only  partially  subdivided.  The  four  diuriN  on 
the  little-toe  side  (homologous  with  the  Little-finger 
side  of  the  hand)  had  four  normal  dibits,  none  of  which 
is  a  great  toe.  The  five  toes  of  tin-  great  tor  side  had 
the  largest  digit,  much  like  a  great  tor.  on  tin-  outside. 
The  third  and  fourth  digits  of  this  side  are  only  partially 
divided.  This  looks  like  another  case  of  symmetry 
reversal  on  the  part  of  the  component  of  the  w< 
side  back  to  the  specific  asymmetry  of  the  -ingle  limb. 


204  THE  PHYSIOLOGY  OF  TWINNING 

It  seems  highly  probable  that  all  vertebrates  undergo 
processes  of  bilateral  doubling  such  as  these  described. 
Essentially  the  same  conditions  have  been  shown  to 
apply  in  many  cases  among  the  arthropods.  These 
and  the  vertebrates  are  the  only  groups  that  have  well- 
defined  limbs.  I  am  aware  that  our  attempt  to  effect 
an  analysis  of  the  problem  of  symmetry  reversal  is  not 
an  unqualified  success,  but  I  am  convinced  that  this 
point  of  view  has  elements  of  value  and  will  help  in  the 
ultimate  solution  of  a  very  perplexing  problem. 

THE   CAUSES   OF   LIMB-DOUBLING 

If  limb-doubling  is  equivalent  to  that  of  bilateral 
twinning  it  should  have  the  same  general  causes.     It  is 
our  present  theory  that  limb-doubling  is  caused  by  a 
lowering  of  the  developmental  rate  at  the  time  when 
the  limb-bud  is  beginning  its  process  of  outgrowth  or 
before  the  terminal  elements  have  become  differentiated. 
Now  what  evidences  have  we  at  present  that  limb- 
doubling  is  associated  with  developmental  retardation? 
There  are  two  independent  lines  of  evidence.     The  first 
is  that  double  limbs  most  frequently  occur  in  connection 
with  regeneration  processes  in  which  we  know  that, 
prior  to  regrowth,  there  takes  place  a  nearly  complete 
dedifferentiation  of  tissues  and  a  loss  of  axiate  organiza- 
tion.    It  commonly  happens  in  regeneration  experiments 
that  two  heads  grow  out  in  place  of  one  and  similarly 
the  embryonic  rudiment  of  a  regenerating  limb  is  likely 
to   lose  its   unity   and   become   double.     Regeneration 
processes  are  usually  less  rapid  in  their  early  stages 
than  are  normal  developmental  processes,  and  require  a 
reaxiation  process  before  they  can  go  ahead.    The  second 


TWINNING  (DUPLICITY)  IX  LIMB  205 

piece  of  evidence  is  of  an  entirely  different  sort  and  ari 
out  of  data  furnished  by  Detwiler  (1920).     Using  the 
methods  of  Harrison,  previously  referred  to,  this  author 
transplanted    limb-buds    of    Amblystoma    from     their 

normal  position  to  various  levels  more  posterior.  Many 
of  these  transplanted  limb-buds  underwent  doubling 
and  showed  mirror-image  symmetry.  According  to 
Detwiler,  "  there  occurred  a  gradual  increase  in  the 
number  of  reduplications  as  the  limbs  became  trai 
planted  farther  and  farther  away  from  the  normal 
situation."  If  we  take  this  statement  in  connection  with 
another  series  of  facts,  it  becomes  significant;  for  the 
farther  away  from  the  normal  position  the  graft  is  placed 
the  slower  it  is  to  develop.  Thus  we  have  the  greatest 
frequency  in  limb-doubling  when  the  developmental  rate 
of  the  limb-bud  is  most  retarded;  and  the  causal  basis  of 
limb-doubling  is  seen  to  be  in  all  probability  the  same 
as  in  other  more  typical  forms  of  twinning. 


CHAPTER  XV 
TWINNING  AS  A  MODE  OF  REPRODUCTION 

In  view  of  the  confusion  that  prevails  at  present 
regarding  twinning  processes  I  believe  that  the  following 
attempt  to  classify  the  various  modes  of  reproduction, 
including  the  several  types  of  twinning,  will  help  to 
clarify  the  situation. 

One  school  of  writers  finds  the  broadest  distinction 
among  modes  of  reproduction  to  lie  in  whether  they  are 
sexual  or  asexual.  All  types  of  reproduction  in  which 
there  is  no  union  of  gametes  are  lumped  together  into 
the  category  of  agametic  or  asexual  reproduction  as 
over  against  gametic  or  sexual  reproduction. 

In  his  textbook,  Principles  of  Zoology,  Shull  (1920) 

expresses  himself  as  follows: 

Asexual  or  non-sexual  reproduction  includes  all  those  methods 
of  reproduction  which  require  but  a  single  parent  for  the  reproduc- 
tion of  offspring  and  do  not  involve  germ  cells.  Sexual  reproduc- 
tion as  a  rule  involves  two  parents  and  the  production  of  two  kinds 
of  germ  cells,  the  eggs  and  sperms.  It  is  usually  brought  about 
by  union  of  a  sperm  cell  with  an  egg,  or  less  commonly  by  the 
development  of  the  egg  without  union  with  the  sperm. 

He  then  proceeds  to  classify  modes  of  reproduction 
essentially  as  follows: 
I.  Asexual  reproduction 

s  _,    ...      [external 

a)  Budding  (internal 

,N   „.    .      /longitudinal 

b)  Fission  ltransverse 

c)  Sporulation 

206 


TWINNING  AS  A  MODE  OF  REPRODUCTION     207 

II.  Sexual  reproduction 

a)  In  Protozoa  < .    r      > 

'  ^heterogamy 


b)  In  Metazoa< 


"the  typical  fertilization  pro. 
parthenogenesis 
paedogenesis 
hermaphroditism 


Such  a  classification,  while  in  most  respects  Logical, 
is  incomplete,  and  fails  to  take  account  of  mil- 
twinning,  and  polyembryony.  At  the  risk  of  being 
considered  venturesome,  I  wish  to  present  for  consider- 
ation another  classification  of  modes  of  reproduction 
that  seems  to  me,  in  some  respects  at  least,  more  workable 
than  those  previously  proposed. 

MODES  OF  REPRODUCTION 
I.  Axiate  reproduction 

a    tt  •    11   1     fa)  Mitosis  in  both  Protozoa  and  Mcta/0.1 

A.  Unicellular^  Amitosis  in  both  Protozoa  and  Meta/, 

B.  Multicellular 

^  r    .       (a)  In  embryos 

1.  Transverse  fission  |6)  In  adult; 

2.  Lateral  budding 

a)  Fission    of    blastoderm    before    axis 
symmetry  is  laid  down 

b)  Production  of  two  (or  more)  axes  through 
plural  gastrulation 

c)  Bilateral  fission  of  the  right-  ami  Left-hand 
primordia  of  a  single  embryonic  axis 

II.  Xon-axiate  reproduction 

(a)  Gemmulation 
b)  Stotoblast  formation 
c)   Sporulation 

a)  Parthenogenetic 
i.  Paedogenetic 

ii.  Adult 

b)  Syngamous 

i.  Isogamoa.-. 
ii.  Heterogamous 


3.  Twinning  < 


B.  Gametic 


208  THE  PHYSIOLOGY  OF  TWINNING 

I.      AXIATE   REPRODUCTION 

The  characteristic  feature  of  axiate  reproduction 
is  that  it  involves  a  separation,  either  complete  or 
incomplete,  of  a  single  axiate  individual  into  two  or 
more  individuals  in  such  a  precise  fashion  that  the  line 
of  separation  between  the  two  products  of  division  bears 
a  definite  relation  to  at  least  one  of  the  structural  or 
functional  axes  of  the  original  individual.  There  are, 
according  to  Child,  three  types  of  axiate  organization 
among  organisms,  which  are,  in  the  order  of  their  evo- 
lutionary origin  and  their  ontogenetic  development:  (a) 
the  radial  axis  or  that  involving  a  gradient  of  metabolic 
activity  from  the  center  to  the  surface;  (b)  the  axis  of 
polarity,  involving  a  gradient  of  metabolic  activity 
running  from  anterior  to  posterior  end  or  from  animal 
to  vegetal  pole  of  single  cells;  (c)  the  axis  of  symmetry, 
involving  a  double  gradient  running  laterally  from  either 
median  dorsal  or  median  ventral  sides,  and  giving  rise 
to  bilateral  symmetry. 

There  is  a  type  of  axiate  reproduction  associated 
with  each  of  these  three  types  of  axiate  organization. 

A.      UNICELLULAR  AXIATE   REPRODUCTION 

Doubtless  the  most  primitive  as  well  as  the  most 
universal  type  of  axiate  reproduction  is  that  exhibited 
in  the  reproduction  of  cells.  All  cells,  no  matter  how 
little  differentiated  they  may  be  in  other  respects,  have 
at  least  a  center-surface  organization  and  it  is  this  axis 
that  determines  the  sequence  of  events  seen  in  both 
mitotic  and  amitotic  cell  division  whether  in  Protozoa 
or  in  Metazoa.  In  this  type  of  reproduction  the  essential 
feature  is  that  division  or  doubling  of  cell  structures 


TWINNING  AS  A  MODE  OF  REPRODU<  1  [ON     209 

begins  with  those  occupying  the  dynamic  center  of  the 
cell  and  proceeds  from   the   center   to   the   periphery, 

culminating  in  a  constriction  of  the  cortex  and  the  cell 
membrane.  In  amitotic  division  the  nucleolus,  originally 
single,  divides  into  two  nucleoli,  which  migrate  apart 
to  opposite  sides  of  the  nucleolus,  thus  becoming  physio- 
logically isolated;  the  nucleus  then  divides  and  the  two 
halves  migrate  apart;  and  finally  the  cytoplasm  becomes 
at  first  physiologically,  and  later  physically,  separated 
or  isolated  into  two  independent  masses.  In  mitosis 
the  centrosome  and  astrophere  seem  to  constitute  the 
dynamic  center  of  the  cell  and  to  take  the  initiative  in 
cell-reproduction.  Two  centers  arise  instead  of  one  and 
these  migrate  apart,  each  organizing  about  itself  a  radial 
system.  The  chromosomes,  at  first  occupying  a  neutral 
position  between  the  two  radial  systems,  divide  longi- 
tudinally so  that  half  of  each  chromosome  migrates  to 
each  daughter-system;  and  finally  the  cytoplasm  is 
partitioned  off  into  two  independent  or  semi-independent 
systems  and  cell-reproduction  has  been  accomplished. 

Cell-division  in  both  Protozoa  and  Metazoa  may  be 
either  equational  or  differential.  When,  as  in  the 
fission  of  the  Mastigophora  and  in  the  majority  of  cas 
of  early  cleavage,  the  line  of  separation  (cleavage 
furrow)  is  meridianal,  i.e.,  parallel  to  the  axis  of  polarity 
of  the  cell,  the  cell  products  are,  as  a  rule,  equivalent, 
each  being  a  mirror-image  of  the  other.  This  may 
justly  be  termed  cellular  twinning,  for  it  strikingly 
resembles  the  most  characteristic  type-  of  twinning  seen 
in  the  higher  organisms  and  differs  only  in  that  the 
process  is  concerned  with  but  one  cell  instead  of  many. 
When,  however,  the  line  of  separation  is  at  right  angles 


210  THE  PHYSIOLOGY  OF  TWINNING 

to  the  axis  of  polarity,  as  it  is  in  the  majority  of  free 
Infusoria  and  in  the  first  cleavage  of  Ascaris,  Unio,  and 
similar  forms,  the  process  is  much  more  like  the  transverse 
fission  seen  in  natworms  and  in  metameric  groups. 
It  differs  from  twinning  in  that  the  daughter-cells  are 
not  equivalent  and  in  a  lack  of  mirror-imaging  between 
them. 

B.      MULTICELLULAR  AXIATE   REPRODUCTION 

i.  Transverse  fission.— This  type  of  reproduction 
involves  the  cutting  off  of  a  new  individual  by  means 
of  a  fission  plane  at  right  angles  to  the  primary  axis. 
As  a  rule  a  new  organism  is  cut  off  from  the  basal  region 
of  an  individual  which  is  elongated  in  and  growing 
in  a  posterior  or  basal  direction.  Transverse  fission  is 
mainly  characteristic  of  free-living  as  opposed  to  sessile 
organisms,  in  which  a  posterior  zooid,  if  cut  off,  is  free 
to  break  away  and  lead  an  independent  life.  Examples 
of  transverse  fission  are  seen  in  the  formation  of  new 
zooids  in  planarians,  in  strobilation  in  the  cestodes, 
and  in  metameric  segmentation  in  the  embryos  and 
larvae,  of  annelids,  arthropods,  and  vertebrates.  In  the 
more  primitive  types  of  transverse  fission  the  newly 
formed  individual  breaks  away  from  the  old  and  regener- 
ates a  new  anterior  end  or  head,  becoming  ultimately 
a  complete  individual.  In  the  case  of  strobilation  the 
individuals  produced  by  fission,  though  they  sooner  or 
later  become  independent,  are  never  able  to  regenerate 
the  head,  and  hence  remain  incomplete.  In  metameric 
segmentation  the  new  individuals  are  incompletely  cut 
off  and  remain  attached,  as  subordinate  zooids,  to  the 
original  head,  and  the  whole  series  becomes  secondarily 
integrated  into  a  single  organism.     Later  on  some  of 


TWINNING  AS  A  MODE  OF  REPRODUCTION 


21  I 


these  metameres  specialize  so  as  to  become  sexually 
mature  and  gametic  reproduction  takes  plai  In  a  very 
real  sense,  therefore,  the  process  of  segmental  Jon  may 
be  said  to  represent  an  asexual  phase  in  the  life  i  ycle  of 
a  metameric  animal. 

2.  Lateral  budding  is  the  characteristic  asexual 
method  of  reproduction  of  sessile  or  sedentary  organisms 
in  which  one  end  of  the  primary  axis  is  permanently  or 
semi-permanently  attached  to  the  substratum.  We 
find  budding  of  this  kind  in  Porifera  and  Coeleriterata, 
the  two  lowest  metazoan  phyla  in  which  there  is  no  axis 
of  bilateral  symmetry.  The  characteristic  feature  of 
this  type  of  reproduction  is  that  at  some  level  of  the 
primary  axis  a  new  growing-point  arises  that  has  acquire  1 
an  independence  from  the  dominance  of  trfe  apical  end. 
This  new  apical  point  grows  out  essentially  at  right 
angles  to  the  main  axis,  although  subsequent  flexures 
may  alter  the  angle  between  the  two  axes.  The  new 
growing-point  becomes  the  apical  end  of  a  new  zooid  and 
this  proceeds  to  organize  its  own  basal  parts.  In  the 
coelenterates  the  formation  of  a  series  of  asexual  zooids 
into  a  colony  is  essentially  like  what  happens  in  (he  case 
of  strobilation  or  in  that  of  metameric  segmentation. 
In  both  cases  the  younger  zooids  remain  sexless,  but  in 
later  stages  at  least  some  of  the  zooids  become  sexually 
mature  and  produce  gametes. 

3.  Twinning. — Twinning    is    a    precocious    form 
axiate  reproduction  involving  a  physiological  isolation  of 
bilaterally  symmetrical  halves  of  the  blastoderm  and  the 
consequent  physical  separation  of  a  single  individual  into 
two  equivalent  parts.     Gross  fragmentation  <>t"  a  blast 
derm  without  reference  to  axiate  relations,  even  it  it  ,^i\ 


212  THE  PHYSIOLOGY  OF  TWINNING 

rise  to  several  independent  embryos,  is  not  twinning. 
Twinning  is  essentially  a  dichotomy,  a  division  of  one 
primordium  into  two.  Repeated  dichotomies  may  give 
rise  to  numerous  offspring  from  a  single  blastoderm  as  in 
the  South  American  armadillo,  Dasypus  hybridus;  or  there 
may  be  only  two  dichotomies  as  in  Dasypus  novemcinctus , 
or  there  may  be  one  complete  dichotomy,  followed  by  a 
partial  dichotomy  of  one  twin,  as  in  certain  cases  of 
so-called  triple  monstrosities  in  fish.  Twinning  is  essen- 
tially a  physiological  isolation  of  two  equivalent  growing- 
points  due  to  a  partial  loss  of  integration  of  the  bilateral 
halves  of  the  blastoderm.  The  completeness  or  incom- 
pleteness of  the  process  depends  upon  the  degree  to  which 
the  dominance  of  the  original  apical  end  has  been  sup- 
pressed. Twinning  differs  primarily  from  both  trans- 
verse fission  and  lateral  budding  in  that  it  is  an  affair 
of  the  axis  of  symmetry,  instead  of  the  axis  of  polarity. 

II.      NON-AXIATE   REPRODUCTION 

This  type  of  reproduction  has  apparently  no  direct 
reference  to  the  axiate  relations  of  the  parent  organism. 
Small  masses  of  tissues  or  single  cells  are  isolated  in 
various  ways  from  the  parent  tissues — usually  internal 
tissues — and  are  capable  under  the  proper  environmental 
conditions  of  reproducing  new  organisms  like  the  parent. 
It  is  probably  true  that  the  tissues  that  give  rise  to 
gemmules,  statoblasts,  spores,  or  gametes,  are  so  related 
to  the  axes  of  the  parents  that  physiological  isolation 
is  favored.  In  that  sense  we  might  consider  all  repro- 
duction as,  in  last  analysis,  axiate,  but  for  our  purposes 
the  axiate  relation  is  so  vague,  if  present  at  all,  that  we 
are  justified  in  ignoring  its  existence. 


TWINNING  AS  A  MODE  OF  REPRODUCTION 

i.  Agametic  non-axiate  reproduction.     Little  need  be 

said  about  the  modes  of  reproduction  included  in  this 
category.  They  have  sometimes  been  considered  as 
cases  of  internal  budding,  because  groups  of  cells  are 
cut  off  instead  of  single  cells  as  in  gametii  reproduction. 
There  are  probably  various  gradations  between  agametii 
and  gametic  reproduction.  In  the  case  of  polyembryony 
in  the  parasitic  hymenoptera  it  seems  to  be  true  that  at 
first  small  groups  of  cells  become  isolated  from  the 
embryo  or  polygerm  and,  after  two  or  three  generations 
of  somewhat  gross  fragmentation  of  the  polygerm,  single 
cells  become  isolated  and  develop  into  the  definiti 
sexual  embryos.  Thus  polyembryony  seems  to  involve 
both  agametic  and  gametic  non-axiate  reproduction. 

2.  Gametic  reproduction.— The  characteristic  feature 
of  this  type  of  reproduction  is  that  single,  more  or  less 
specialized,  cells  (gametes)  are  formed  which,  on  isolation, 
are  capable  of  reproducing  a  whole  organism.  The 
period  at  which  gametes  may  be  isolated  varies  greatly 
in  different  groups  of  animal-.  When  the  isolation 
takes  place  in  an  embryonic,  larval,  or  juvenile  stage, 
we  speak  of  the  condition  as  paedo genesis.  One  of  tin- 
most  interesting  life-cycles,  involving  very  early  paedo- 
genesis,  is  that  of  the  liver  fluke.  In  this  species  of 
parasitic  flatworm  the  very  young  ciliated  larva,  the 
miracidium,  bores  its  way  into  the  soft  tissues  ^i  a  snail 
and  there  grows  into  a  bag-like  vesicle  called  the  sporocyst 
The  sporocyst  produces  within  its  cavity  a  number  of 
cells  which  behave  exactly  like  parthenogenetic  garnet 
for  each  cell  undergoes  maturation  and  goes  through 
a  regular  process  of  cleavage  resulting  in  a  larva  of  a 
different   sort,   called    a    redia.     These    redias    in    turn 


214  THE  PHYSIOLOGY  OF  TWINNING 

reproduce  much  in  the  same  way  for  some  generations 
until  finally  a  generation  of  cercarias  is  produced. 
These  find  their  way  out  of  the  snail  and  into  the  liver  of 
the  sheep,  where  each  slowly  transforms  itself  into  an 
adult  fluke. 

There  are  several  species  of  paedogenetic  insects  in 
which  reproduction  takes  place  in  the  larval  condition. 
Paedogenesis  is  recognized  in  some  of  the  vertebrates 
also,  as  in  the  classic  Axolotl  and  probably  in  the  perenni- 
branchiate  urodeles.  In  fact  paedogenesis  grades  over 
almost  imperceptibly  into  full  adult  reproduction.  In 
very  early  paedogenesis  the  reproduction  is  of  necessity 
parthenogenetic,  but  in  later  paedogenesis  regular  syn- 
gamous  reproduction  takes  place.  Just  as  there  is  a 
gradation  between  agametic  and  gametic  reproduction,  so 
we  have  a  graded  series  of  stages  ranging  from  extremely 
early  paedogenesis  to  late  adult  syngamous  reproduction. 
Parthenogenesis  is  evidently  a  phase  of  gametic  reproduc- 
tion and  its  incidence  is  capricious;  we  find  it  here  and 
there  throughout  the  animal  kingdom  from  Protozoa 
to  Chordata.  It  is  not  to  be  thought  of  as  a  reversion 
to  a  primitive  mode  of  reproduction  but  rather  as  an 
evidence  of  racial  senescence. 

THE  COMMON  FEATURE  OF  ALL  REPRODUCTIVE 

MODES 

The  one  connecting  link  between  the  various  modes 
of  reproduction  is  the  principle  of  physiological  isolation. 
So  long  as  the  individual  is  completely  integrated  in  all 
of  its  parts  it  will  not  reproduce,  but  if  for  any  reason 
the  integrative  forces  that  hold  together  the  various 
parts  into  a  single  organism  weaken,  either  a  general  or 


TWINNING  AS  A  .MODI,  oi    REPRODUi   ll<).\     215 

a  local  breaking  away  or  emancipation  of  part 

and  the  isolated  parts  become  the  beginnings  of  ne 

organisms.  If  the  integrative  fon  ire  moderately 
lessened  or  inhibited,  only  certain  li  ompletely  int 
grated  or  outlying  parts  tend  to  gain  physiological 
independence.  Functioning  independently  tends  further 
to  isolate  and,  in  the  end,  the  part  becomes  not  only 
physiologically,  but  physically  independent.  Then 
follows  the  process  of  reconstituting  the  specific  form, 
and  this  is  able  to  take  place  whether  the  isolated  part 
is  a  single  germ  cell,  a  small  ma>s  of  internal  cells,  a 
lateral  bud,  the  isolated  halves  of  a  bilateral  primordium, 
or  the  posterior  half  of  an  axiate  animal. 

TWINNING   AND   ALTERNATION   OF    GENERATION 

Among  the  lower  animals  it  is  very  common  to  find 
that  the  life-cycle  is  much  more  complex  than  in  the 
higher  animals.  In  the  colonial  hydroids,  for  example. 
an  egg  develops  a  young  hydranth  which  by  lateral 
budding   produces   a   whole   series    of    similar  ial 

hydranths.  Late  in  the  season  certain  budded  individ- 
uals, medusa  buds,  develop  into  free-swirnming  sexual  in- 
dividuals, medusae,  and  these  produ*  <  eggs  and  sperm, 
which  in  turn  unite  to  give  us  the  fertilized  egg  again. 
We  thus  seem  to  have  an  alternation  between  an  ial 

mode  of  reproduction  and  a  sexual,  which  has  been  called 
alternation  of  generations.  Without  seriously  opposing 
the  value  of  thus  marking  off  what  is  essentially  a 
continuous  ontogeny  into  separate  generations,  I  wish 
to  enter  a  protest  against  what  appears  to  me  t.»  be 
misuse  of  the  facts  of  twinning,  especially  that  in  the 
armadillos.     Twinning  is,  in  my  opinion,  a  cenogenetic 


216  THE  PHYSIOLOGY  OF  TWINNING 

or  a  senescent  condition  and  in  no  sense  a  reversion  to  an 
ancestral  condition.  Therefore  it  is  a  mistake  to  consider 
that  twinning  in  any  sense  represents  the  now  generally 
lost  asexual  phase  in  the  life-cycle  of  the  vertebrates. 

Gemmill  (191 2)  is  doubtless  the  originator  of  this 
mistake  as  may  be  judged  from  the  following  passage: 

The  view  has  often  been  suggested  that  the  blastoderm  may 
be  looked  upon  as  a  stock,  able  to  give  rise  vegetatively,  so  to 
speak,  to  more  than  one  embryo.  The  natural  comparisons  have 
been  drawn  between  this  faculty  and  the  alternation  of  generations 
which  occurs  normally  in  some  groups  of  lower  animals  and  in 
plants.  It  has  been  sought  to  recognize  alternation  of  generations 
in  the  development  of  all  animals.  More  probably,  however,  in 
animals,  twinning,  double  and  triple  monstrosity,  polyembryony, 
and  alternation  of  generations,  provide  instances  in  which  a  common 
"potentiality"  has  become  realized,  and  beyond  that  are  not  neces- 
sarily connected  by  any  nexus  of  a  direct  or  phylogenetic  character. 

This  is  at  least  a  non-committal  statement  of  this 
point  of  view,  but  it  has  evidently  led  to  a  considerably 
less  cautious  statement  by  Stockard,  who  says: 

The  suggestion  has  frequently  been  made  that  the  blastoderm 
may  be  looked  upon  as  a  stock  able  to  give  rise  asexually  to  more 
than  one  embryo.  Since  the  natural  process  of  budding  to  form 
four  or  more  embryos  in  the  armadillo  is  recognized,  and  accessory 
individuals  may  be  produced  experimentally  from  other  vertebrate 
eggs,  it  becomes  evident  that  even  man  and  the  highest  animals 
may  actually  at  times  exhibit  an  alternation  of  the  sexual  and 
asexual  processes  of  reproduction. 

If  any  real  significance  is  to  be  attributed  to  finding 
among  vertebrates  a  similitude  of  the  so-called  alterna- 
tion of  generations  of  the  lower  forms,  one  need  hardly 
go  so  far  afield  to  discover  it.  Instead  of  tracing  it 
back  to  a  typically  sessile  and  radial  group  such  as  the 
Coelenterates,  we  can  find  it  among  the  Rhabdocoels, 


TWINNING  AS  A  MODE  OF  REPRODUCTION     217 

a  free-living  group  with  bilateral  symmetry.  Su<  h  types 
as  the  planarians  and  Microstomum  illustrate  alternation 
of  generations  very  beautifully.  For  a  long  timi 
while  relatively  young,  they  reproduce  by  trans 
fission  and  after  a  series  of  fissions  reproduci  ually. 
In  the  annelids  first,  and  then  in  the  vertebrates 
have  the  equivalent  of  the  asexual  period  (characterized 
by  transverse  fission)  in  the  process  of  metameric 
segmentation.  The  agamic  period  is  pushed  farth 
and  farther  back  in  the  life-cycle  until  in  the  high 
vertebrates  it  is  completed  during  the  firsl  few  days  of 
development.  The  introduction  of  cases  of  embryonic 
twinning  in  order  to  supply  the  missing  link  in  an 
ideally  universal  alternation  of  generations  is,  therefore, 
to  say  the  least,  gratuitous.  Moreover,  if  this  view 
were  carried  out  to  its  logical  conclusion,  we  would  be 
led  to  admit  that  wherever  we  find  cases  of  doubling  of 
normally  single  individuals  or  parts  of  individuals  we 
have  reminiscences  of  a  lost  asexual  generation.  Such 
a  theory  would  undoubtedly  seem  absurd  if  applied  to 
cases  of  doubling  of  tails  or  doubling  of  Limbs,  which 
begin  at  relatively  late  stages  of  ontogeny.  It  seems 
just  as  absurd  to  designate  the  partial  fission  of  the 
bilateral  primordia  of  a  single  embryonic  axis  as  a 
reminiscence  of  a  lost  asexual  generation.  \  et  such 
processes  are  due  essentially  to  the  same  factors  as  is 
twinning  in  the  armadillo.  At  the  risk  oi  ming 
pedantic,  therefore,  I  would  once  more  suggest  to 
biologists  the  need  of  caution  in  assigning  phylogenetic 
values  to  cenogenetic  processes,  such  .1-  one  twin- 
ning in  armadillos  and  in  man,  or  to  experimentally 
induced  twinning  in  fishes. 


218  THE  PHYSIOLOGY  OF  TWINNING 

POLYEMBRYONY  AND   TWINNING 

It  is  an  unfortunate  circumstance  that  one-egg 
twinning  in  the  armadillos  was  described  first  as  a  case  of 
polyembryony.  The  fact  that  all  embryos  derived  from 
a  single  egg  were  of  the  same  sex  strongly  reminded  the 
discoverers  of  twinning  in  the  armadillos  of  the  condition 
described  by  Sylvestri  and  others  for  the  parasitic 
hymenoptera,  where  sometimes  hundreds  of  embryos 
were  produced  from  a  single  egg  and  all  embryos  from 
any  one  egg  were  of  the  same  sex.  Since  the  hymen- 
opteran  condition  had  with  considerable  appropriateness 
been  called  polyembryony,  it  was  not  unnatural  that 
several  of  us  who  first  studied  the  armadillo  situation 
should  adopt  for  it  too  the  term  polyembryony.  At  that 
time  we  did  not  fully  understand  the  exact  mode  of  origin 
of  the  multiple  embryos  in  either  parasitic  hymen- 
opteran  or  in  the  armadillo.  Now  that  both  conditions 
are  adequately  understood,  it  is  clear  that  the  armadillo 
case  is  a  clear-cut  instance  of  dichotomous  twinning, 
a  form  of  axiate  reproduction;  while  true  polyembryony, 
as  seen  in  such  forms  as  Paracopidomopsis  (Patterson, 
1 921)  involves  nothing  at  all  equivalent  to  the  processes 
of  twinning,  but  follows  a  mode  of  reproduction  essen- 
tially non-axiate,  resembling  much  more  closely  the 
paedogenetic  type  of  reproduction  of  the  liver  fluke 
than  it  does  twinning  in  the  armadillos.  In  Paracopi- 
domopsis there  are  evidences  of  several  embryonic 
generations  that  remind  one  of  the  several  larval  genera- 
tions in  the  liver  fluke,  Fasciola.  The  cleavage  period 
results  in  a  so-called  "  morula  stage, ':  which  loses  its 
axiate  relations  and  becomes  a  mass  of  generalized  cells 
called  a  "polygerm."     This  soon  becomes  subdivided 


TWINNING  AS  A  MODE  OF  REPROD1  CTION 

into  fifteen  or  twenty  primary  cell  ma  Since  there 

is  evidently  some  integration  among  the  cells  of  th< 

primary  cell  masses,  they  may  be  considered  as  embry 
of   the  first  asexual  generation.     The  primary   mas 
soon  lose  their  unity  of  organization  and  break  up  by 
constriction  and  separation  into  secondary  cell  m 
or    embryos    of    a    second    asexual    generation.     This 
happens  once  more  and  tertiary  masses  or  embryos  of 
the  third  asexual  generation  are  formed.     These  tertiary 
embryonic    masses    then    completely    disorganize    into 
components  which  are  the  equivalent  of  single  germ 
cells,  each  of  which  goes  through  a  typical  process  of 
embryonic  development  to  form  an  adult  insect.     It  is 
not  always  possible  to  trace  each  insect  back  to  a  single 
cell,  but  this  was  done  in  enough  cases  to  make  it  practi 
cally  certain  that  each  one  starts  from  a  single  cell. 
Such  a  cell  is  therefore  essentially  a  precociously  formed 
germ    cell    capable    of    parthenogenetic    development. 
How  strikingly  like  the  liver-fluke  case  this  is,  and  h 
unlike  that  of  the  armadillo,   the  reader  may   readily 
ascertain  if  he  studies  the  respective  life-histories  side 
by  side.     Unless,  therefore,  we  are  prepared  to  reduce 
all  twinning  phenomena  to  a  parity  with  the  condition 
in  the  parasitic  hymenoptera  or  even  the  liver  fluke, 
should  cease  to  refer  to  the  mode  of  multiple  embryo 
formation  in  the  armadillo  as  polyembryony  and  call  it 
what  it  really  is — twinning. 


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SCHULTZE,  O. 

1894.     "Die  kunstliche  Erzengung  von  Doppelbildungen  ; 
Froschlarvcn  mit  Hiilfe  abnormer  Gravitationswerk- 
ung,"  Arck.f.  Entw.-Mech.,  Band  5. 

SCHWALBE,  E. 

1907.     Die  Morphologic  dcr  Missbildungcn  dcs  .1/  n  und 

der  Tiere.    II.  Die  Doppelbildungen. 
Shull,  A.  F. 

1920.  Principles  of  Animal  Biology.     New  York. 
Spaeth,  J. 

i860.     "Materiale  vergenommenen  Studien  Qber  Zwillinge 
Zeitschr.  dcr  Gesell.  der  Aerzte  zu  Wien,  Nr.  1  5  u.  1 
Spemann,  II. 

1904.     "  Ueber  experimentell  erzengte  Doppilbeldungen  mil 

cyclopischem    Defect,"    Zobl.     Jahrb..     Supplement, 
Band  7. 

Spemann,  H.,  and  Falkenberg,  H. 

1919-     "t)ber  asyrametrische  Entwicklung  und  Situs  inversus 
viscerum bei Zwillingen  un<l  Doppelbildungen,"    I 
/.  Entw.-Mech.,  Band  45. 
Stockard,  C.  R. 

1921.  "Developmental    Rate    and    Structural    Expression: 
An  Experimental  Study  of  Twins.  'Double  Monst< 
and   Single   Deformities,   etc.."   Atner.   Jour. 
XXVIII. 

Streeter,  G.  L. 

1919-     "Formation  of  Single-Ovum   Twins."  John*  II  : 
I  I  os  p.  Bull.,  XXX. 
Sumner,  F.  B. 

1904.    "A  Study  of  Early  Fish  Development,  Experimental 
and  Morphological,"  Arch.f.  lint:,-.  \!         \\  1 1. 

SWETT,  F.   II. 

1921.    "Situs   viscerum    in    Double    ln.ut,"    Anal.   R 
XXII. 


226  THE  PHYSIOLOGY  OF  TWINNING 

Tannreuther,  G.  W. 

191 9.     " Partial  and  Complete  Duplicity  in  Chick  Embryos," 
Anat.  Record,  XVI. 
Tornier,  G. 

1896.  "Uber  Hyperdactylie,  Regeneration  und  Vererbung 
mit  Experimenten,"  Arch.  f.  Entw.-Mech.,  Band  3. 

1897.  "Uber  experimentell  erzeugte  dreischwanzige  Ei- 
dechsen,  und  Doppelgleidmassen  von  Molchen,"  Zool. 
Anz.,  Band  20. 

1900.     "Neues  iiber  das  natiirliche  Entstehung  und  experi- 
mentelle  Erzeugung  uberzahliger  und  Zwillingsbild- 
ungen,"  ibid.,  Band  24. 
Vejdovsky,  Fr. 

1888-92.     Entwicklungsgeschichte  Untersuchungen.     Prag. 
v.  Baer,  K.  E. 

1845.     "Ueber   doppeleibige   Missgeburten   oder  organische 
Verdoppelungen  in  Wirbeltieren,"  St.  Petersburg  Mem. 
Acad.  Set. j  Ser.  6,  No.  4. 
Weber,  Roxie  A. 

191 7.     "Observations  on  the  Structure  of  Double  Monsters 
in  the  Earthworm,"  Biol.  Bull.,  XXXIII. 
Welch,  P.  S. 

192 1.     "Bifurcation   in   Embryos  of  Tubifex,"  Biol.  Bull., 
XLI. 
Wilder,  H.  H. 

1904.     "Duplicate   Twins    and    Double    Monsters,"    Amer. 

Jour.  Anat.,  III. 
1908.     "The  Morphology  of  Cosmobia;    Speculations  Con- 
cerning the  Significance  of  Certain  Types  of  Mon- 
sters," ibid..  VIII. 
Wilson,  E.  B. 

1893.     "Amphioxus   and   the   Mosaic  Theory   of  Develop- 
ment," Jour.  Morph.,  VIII. 
Windle,  B.  C.  H. 

1895.  "On  Double  Malformations  Amongst  Fishes,"  London 
Prcc.  Roy,  Soc. 


INDKX 


Acardia,  149-54 

Acardii:  acephali,  153;  acormi, 
152,  154;  amorphi,  153,  154; 
completi,  151 

Agametic  non-axiate  reproduction, 
213 

Allolobo phora  foetida,  30;  A.  sub- 
rubicunda,  31,  32;  A.  trap- 
ezoidcs,  30-32 

Alternation  of  generations  and 
twinning,  216,  217 

Amblystoma,  194-200 

Amphioxus,  twinning  in,  98,  99 

Anadidymi:  in  birds,  83;  in 
fishes,  43,  45,  46;  in  starfish 
larvae,  23,  24;  in  turtle,  95 

Androvandus,  U.,  38 

Arey,  L.  B.,  130-32 

Assheton,  108,  129 

Autosite  and  parasite  twins:  in 
birds,  87,  88;  in  fishes,  45,  46; 
in  man,  155;  in  starfishes,  26,  27 

Axiate  reproduction,  208-12 

Axolotl,  paedogenesis  in,  214 

Barbieri,  C,  42 
Barfurth,  98 

Bateson,  W.,  3,  94,  95,  193,  194, 

200-202 

Bellamy,  A.  \Y.,  viii,  93 
Bilaterality  in  echinoderm  larvae, 

179-83  ' 
Blastotomy:      physiological,      t6; 

theory  of  twinning,  1 

Bruch,  95 

Bryce-Teacher  ovum,  1  28 

Bryophyllum,  62 

Budding:  theory  of  twinning,  55, 
108-20;  lateral,  2  \  1 


Chelonia,  05  07;  breeding  habit- 
of,  95,  q6j  minimal  twinning  in. 
96-97 

Child,  C.  M..  6i 

Concrescence  theory,  and  inter- 
pretation of  twinning,  55 

Conjoined  twins:   classification 

in  fishes,  43,  44,  51-72;    in- 
influences  of,  in  man,    15.; 
mode    of  origin,   in    man,  1 
26 

Cosmobia:  in  birds,  82  84;  in 
man,  121,  122 

Coste,  M.,  38,  67 

Crowding,  a  cause  of  twinning  in 
Patiria,  15 

Cyclopia,  162,  191 

Cyclopian  monster,  1 24 

Cyclostomata,  98 

Cyprinus  auratus,  191 

Dareste,  C,  7,  73-78, 

Dasypus  hybrid  us,  1 . 

Dasypus  novemcinctus  tea  100, 

212 

Davenport,  C.  B.,  [33, 

Determinate  cleavage,  relation  t<» 
twinning,  3,  4 

Detwiler,  S.  R.,  205 

Dichotomy,  a  synonym  for  twin- 
ning, 1 

I  >iplopagi,  i 

I  dizygotic  twins,  vii 

I  )«ilini.    \ 

Double  hands  and   feet   in   n 

200  202 

Double  limbs,  experimental   | 
duction  of,  [94-  200 


227 


228 


THE  PHYSIOLOGY  OF  TWINNING 


Double  monsters:  interpretation 
of,  6-8;  in  fishes,  51-57;  in 
worms,  29 

Duplicate  twins,  origin  of,  in 
fishes,  46-48 

Duplicity  in  limbs,  193-205 

Duverney,  74 

Dwarf  larvae,  in  Patiria,  15,  16 

Earthworms,  twinning  in,  28,  37 
Echinodermata,  179-85 
Echinus  miliaris,  182-85 
Erdwurm,  Dr.,  132 
Experimental  production  of  twins 
in  fishes,  70-72 

Falkenberg,  H.,  92,  93,  175-78 

Fasciola,  218 

Fisher,  G.  H.,  125 

Fission :  theory  of  origin  of  double 
monsters  in  fishes,  68-70;  theory 
of  twinning  in  armadillo,  108- 
20;   transverse,  210 

Frog  twins,  92 

Fundulns  heteroclitus,  61,  71,  190 

Gametic  reproduction,  213,  214 
Gastrulation,    significance    of,   in 

twinning,  5-7,  11 
Gemmill,  J.  F.,  7,  39,  40,  44,  45, 

49,  5o,  52-54,  56,  59-62,  64,  70, 

7i,  75,  95,  168-70,  216 
Gesell,  A.,  159-63 
Giraldes,  203 
Goldfish  with  double  tails,  191 

Harrison,  R.  G.,  194-200 
Harvey,  W.,  74 

Hazards,    developmental,    in    hu- 
man twins,  135-58 
Heart-dead  twins,  148-54 
Heath,  H.,  21 
Helodrilus  caliginosus  trapezoides, 

Hemiacardia,  149 
Hemididymus,  42 


Hemihypertrophy,  viii,  9,  70,  159- 

63 
Holoacardia,  149 

Homoeosis,  94 

Hybrid  twins,  in  Patiria,  13,  14 

Hyman,  L.  H.,  63,  192 

Indeterminate    cleavage,    relation 
to  twinning,  3,  4 

Jablonowski,  58 

Janus  monsters,  122,  124,  126 

Johnson,  A.,  203 

Kaestner,  S.,  7,  77-80,  85,  98 
Katadidymi:   in  birds,  83,  84,  90; 

in   starfish  larvae,    19,    25;    in 

trout,  42 

Klausner,  F.,  39,  41 

Kleinenberg,  N.,  28-30 

Knoch,  7,  39,  42 

Kopsch,  F.,  39,  42,  935  theory  of 
embryo  formation,  57-60 

Korschelt,  E.,  31,  32 

Lemery,  74 

Lereboullet,  A.,  38,  39 

Lillie,  F.  R.,  viii,  54 

Limb  duplicity,  causes  of,  204,  205 

Liver  fluke,  reproduction  in,  213, 
214,  219 

Lumbricus  terrestris,  30;  L.  trap- 
ezoides, 28 

McBride,  E.  W.,  180,  182-84 
Mateer  ovum,  127,  128 
Meckel,  7,  75 
Mediacardia,  155 
Mesodidymus,  42 
Metameric  segmentation,  210,  217 
Microstomum,  217 
Miller  ovum,  128 
Mirror-imaging,  7,  164,  189 
Monozygotic  twins,  vii,  1 
Monstrorum  Historia,  38 


INDIA" 


Morgan,  T.  H.,  58,  91 

Morrill,  C.  V.,  166,  167,  170,  171 

Mortensen,  184 

Miiller,  J.,  184 

Murray,  J.  J.,  202 

Non-axiatc  reproduction,  212-14 

Ophionotus  hexactis,  184 
Oshima,  H.,  183-85,  203 

Paedogenesis,  213,  214 

Panum,  P.  L.,  7 

Paracopidomopsis,  218 

Parasite  and  autosite  twins:  in 
fishes,  45,  46;  in  starfishes,  26, 
27 

Parthenogenetic  twins,  in  Patiria, 

12,  13 
Patiria  miniata,  n-27,  180-82 

Patterson,  J.  T.,  61,  89,  100-102, 
104, 108-16, 118-20, 127,218,219 

Pelobates  fuscus,  94 

Petromyzon  planeri,  98 

Physiological  isolation:  a  cause  of 
twinning,  5-8,  69,  70;  the 
common  feature  of  modes  of 
reproduction,  214,  215 

Placentae  of  human  one-egg  twins, 
142,  144,  145 

Polyembryony,  1,  9,  103;  versus 
twinning,  218,  219 

Pristiurus,  98 

Pterostichus  miihlfeldii,  193 

Pygopagi,  122,  126 

Rauber,  A.,  7,  38-41 
Reproduction,  modes  of,  206,  207 
Retarded   development,    the    pri- 
mary cause  of  twinning,  14 
Runnstrom,  J.,  184 

Saint-Hilaire :   E.  G.,  75;    I.  '■  . 
Schatz,  F.,  137-58,  i<»5 


Si  limit  t ,  F., 

Schultze,  <  >.,  91 . 

Shull.  A.  I'.,  206,  207 

Siamese  twin 

Situs  inversus  50, 

69,    74,   92,    121,    1 
164-79 

Situs  rarior,  [64 

Situs  solitus.  [64,  \i 

Situs  viscerum  trai 

Solito  inversus,  [64 

Spaeth,  J.,  136,  137,  164 

Spemann,  H.,  92,  175-78 

Spina    bifida:     in    fishes,    42;     in 
frog,  93 

Stenotomus  chrysops,  190 
Stockard,  ('.  K.:   7.  27,  39,  46 
54,  71,  72,  75-  88,  co8,  [32,  1 
139;  theory  about  twinning  ;m<l 
alternation  of  generation 
217;    theory  of  causes  of  twin- 
ning in  armadillo,    103   5,    [io, 
120;   theory  of  origin  of  double 
monsters,  6] 

Streeter,  G.  L.,  120-31 
Strongylocentrotus  lividus, 

Subnormal  development,  caus 

in  twins,  49,  50 

Sumner,  F.  B.,  58 

Swett,  F.  II.,  171 

Sylvestri,  218 

Symmetry,  rules  of,  [94 

Symmetry  reversal:  general  the- 
ory of,  185-89;  in  echinoderm 
larvae,  1 79  85;  in  fish  twins, 
[6  in  human  twins,   1 

68;  in  Triton,  1  75 

Tannreuther,  G.  W 
/  vutogolabrus  adsp*  1 

Third  circulation,  in  pi. unit. 1  of 
human  one  egg  I  •'•  IDS, 

Thoracopa 
Toda,  K.,  viii 
Tornier,  yj 


23° 


THE  PHYSIOLOGY  OF  TWINNING 


Torpedo,  twinning  in,  98 
Triplets:     in    chick,    77,    78,    82; 

rarity  of,  2 
Triton,  92,  175-78 
Twinning:   and  alternation  of  gen- 
erations, 215-17;  a  result  of  loss 
of  polarity;  a  process  of  regula- 
tion, 5-8;    as  a  mode  of  repro- 
duction, 206-19;  causes  of,  5-8; 
causes  of,  in  armadillo,  100-120; 
causes    of,     in    birds,     80-90; 
causes    of,    in    man,     132-34; 
causes  of,  in  Oligochaeta,  36,  37; 
definition  of,  1;    disadvantages 
of,  138,  139;    in  Amphibia,  91- 
94;    in  Amphioxus,  98,  99;    in 
birds,  73-90;    in  Cyclostomata, 
98;    'in  earthworms  and  allies, 
28-37;    in  Elasmobranchii,  98; 
in  fishes,  38-72;   in  limbs,  193- 
205;      in     man,      121-68;      in 
microdrilous    oligochaetes,    32- 
34;    in  Patiria  miniata,   11-27; 
in'Reptilia,    94-975     m    sjF" 
fishes,  n-27;  in  Tubifex  tubifex, 
32-34;    kinds  of,  2;    modes  of, 


in  Oligochaeta,  34,  355    nature 
of,  1 
Twins:    budding  theory  of  origin 
of,  55;   conjoined,  in  fishes,  43, 
45,  46;    frequency  of,  in  fishes, 
38,    39;     human,    121-68;     in- 
fluence of  one  on  another,  25-27; 
modes  of  origin,  in  fishes,  39-49; 
separate,     in     fishes,     43,     44  J 
Siamese,    167;     in   tubal   preg- 
nancy,   130-32;     types    of,    in 
Patiria,  15-22 

Vejdovsky,  F.,  30,  31 
Virchow,  58 

Weber,  R.  A.,  30,  31 

Welch,  P.  S.,  32-34 

Wilder,  H.  H.,  7,  123,  126 

Willier,  B.  H.,  80 

Windle,  B.  C.  H.,  7,  39,  Sh  52, 
61,65 

Winslow,  74 

Wolff,  C.  F.,  7,  74,  75 


>»■'■  ■'  "SJL 


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